WO2003075212A2 - Detecteur de position - Google Patents

Detecteur de position Download PDF

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Publication number
WO2003075212A2
WO2003075212A2 PCT/GB2002/005243 GB0205243W WO03075212A2 WO 2003075212 A2 WO2003075212 A2 WO 2003075212A2 GB 0205243 W GB0205243 W GB 0205243W WO 03075212 A2 WO03075212 A2 WO 03075212A2
Authority
WO
WIPO (PCT)
Prior art keywords
stylus
winding
excitation
phase
frequency
Prior art date
Application number
PCT/GB2002/005243
Other languages
English (en)
Other versions
WO2003075212A3 (fr
Inventor
Gareth John Mccaughan
Original Assignee
Synaptics (Uk) Limited
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority claimed from GB0205116A external-priority patent/GB0205116D0/en
Priority claimed from GB0209372A external-priority patent/GB0209372D0/en
Priority to GB0422083A priority Critical patent/GB2403017A/en
Priority claimed from PCT/GB2002/002387 external-priority patent/WO2002103622A2/fr
Priority claimed from GB0212699A external-priority patent/GB0212699D0/en
Application filed by Synaptics (Uk) Limited filed Critical Synaptics (Uk) Limited
Priority to AU2002343056A priority patent/AU2002343056A1/en
Publication of WO2003075212A2 publication Critical patent/WO2003075212A2/fr
Publication of WO2003075212A3 publication Critical patent/WO2003075212A3/fr

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Classifications

    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
    • G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
    • G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
    • G06F3/041—Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means
    • G06F3/046—Digitisers, e.g. for touch screens or touch pads, characterised by the transducing means by electromagnetic means
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G06F1/32—Means for saving power
    • G06F1/3203—Power management, i.e. event-based initiation of a power-saving mode
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F1/00—Details not covered by groups G06F3/00 - G06F13/00 and G06F21/00
    • G06F1/26—Power supply means, e.g. regulation thereof
    • G06F1/32—Means for saving power
    • G06F1/3203—Power management, i.e. event-based initiation of a power-saving mode
    • G06F1/3234—Power saving characterised by the action undertaken
    • G06F1/325—Power saving in peripheral device
    • G06F1/3262—Power saving in digitizer or tablet
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
    • G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
    • G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
    • G06F3/033—Pointing devices displaced or positioned by the user, e.g. mice, trackballs, pens or joysticks; Accessories therefor
    • G06F3/0354—Pointing devices displaced or positioned by the user, e.g. mice, trackballs, pens or joysticks; Accessories therefor with detection of 2D relative movements between the device, or an operating part thereof, and a plane or surface, e.g. 2D mice, trackballs, pens or pucks
    • G06F3/03545—Pens or stylus
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
    • G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
    • G06F3/03—Arrangements for converting the position or the displacement of a member into a coded form
    • G06F3/033—Pointing devices displaced or positioned by the user, e.g. mice, trackballs, pens or joysticks; Accessories therefor
    • G06F3/0354—Pointing devices displaced or positioned by the user, e.g. mice, trackballs, pens or joysticks; Accessories therefor with detection of 2D relative movements between the device, or an operating part thereof, and a plane or surface, e.g. 2D mice, trackballs, pens or pucks
    • G06F3/03547—Touch pads, in which fingers can move on a surface
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F3/00—Input arrangements for transferring data to be processed into a form capable of being handled by the computer; Output arrangements for transferring data from processing unit to output unit, e.g. interface arrangements
    • G06F3/01—Input arrangements or combined input and output arrangements for interaction between user and computer
    • G06F3/048—Interaction techniques based on graphical user interfaces [GUI]
    • G06F3/0484—Interaction techniques based on graphical user interfaces [GUI] for the control of specific functions or operations, e.g. selecting or manipulating an object, an image or a displayed text element, setting a parameter value or selecting a range
    • G06F3/0485—Scrolling or panning
    • G—PHYSICS
    • G06—COMPUTING; CALCULATING OR COUNTING
    • G06F—ELECTRIC DIGITAL DATA PROCESSING
    • G06F2203/00—Indexing scheme relating to G06F3/00 - G06F3/048
    • G06F2203/033—Indexing scheme relating to G06F3/033
    • G06F2203/0339—Touch strips, e.g. orthogonal touch strips to control cursor movement or scrolling; single touch strip to adjust parameter or to implement a row of soft keys

Definitions

  • the present invention relates to a position sensor and to parts therefor.
  • the invention has particular although not exclusive relevance to the design of windings used in an inductive position sensor.
  • the invention can be used in one, two or three-dimensional position sensors.
  • the sensor is particularly useful for embedding behind the display of a hand-held electronic device such as a personal digital assistant (PDA), mobile telephone, web browser or products embodying combinations of these.
  • PDA personal digital assistant
  • Non-contact linear and rotary position sensors have been proposed for generating, signals indicative of the position of two relatively movable members.
  • one of the members carries one or more sensor windings and the other carries one or more magnetic field generators.
  • the magnetic field generators and the sensor windings are arranged such that the amount of magnetic coupling between the magnetic field generators and the sensor windings varies as a function of the relative position of the two members.
  • the sensor windings and the magnetic field generators are designed to try to make the output signal vary linearly with the relative position between the two members, since this reduces complexity of the signal processing required to determine the positional information.
  • the sensor windings and the magnetic field generator are arranged so that the output signal from the sensor windings varies approximately sinusoidally as a function of the relative position of the two relatively movable members .
  • a particular design of such "sinusoidal" sensor windings is disclosed in the applicant's earlier International application WO 00/33244. Two of the "sinusoidal" type sensor windings are provided which are physically offset from each other so that phase quadrature output signals are provided, from which the position information can be obtained using an arc tangent function.
  • the present invention aims to provide alternative designs of windings.
  • the present invention provides a winding for use in a position sensor, the winding comprising a plurality of series connected loop portions which extend along and transverse to a measurement path, and wherein at least one of the loop portions which extends over a corner of a measurement area is arranged to enclose a greater area than a central portion of the winding.
  • a winding for use in a position sensor, the winding comprising a plurality of series connected loop portions which extend along and transverse to a measurement path and wherein the winding comprises means for increasing the sensitivity of the winding to electromagnetic signals at one or more edges of a measurement area which extend along the measurement path.
  • a complete x-y digitising tablet system which incorporates the winding discussed above.
  • a new stylus is described together with novel processing electronics for processing the signals obtained from the windings. Whilst these new windings can be used with the new stylus and the new processing electronics, it is not essential. Similarly, the stylus and processing electronics described in the main embodiment may be used with other types of windings.
  • Figure 1 is a schematic view of a hand-held personal digital assistant (PDA) which includes an x-y digitising system located behind the PDA's liquid crystal display which can sense the (x,y) position of a resonant stylus;
  • PDA personal digital assistant
  • Figure 2 schematically illustrates a cross-sectional view of the personal digital assistant shown in Figure 1, illustrating the positional relationship between a sensor printed circuit board of the digitising system and the liquid crystal display;
  • Figure 3a is a schematic functional block diagram illustrating the excitation and processing electronics of the x-y digitising system and illustrating the magnetic coupling between an excitation winding of the digitising system and the resonant stylus and the magnetic coupling between the resonant stylus and four sensor windings which form part of the digitising system;
  • Figure 3b is a timing plot illustrating the form of various signals within the x-y digitising system shown in Figure 3a during an excitation and receive cycle;
  • Figure 4a schematically illustrates an approximation of the way in which the peak amplitude of the signals induced in x-sensor windings of the digitising system vary with the x-coordinate of the position of the stylus relative to the liquid crystal display;
  • Figure 4b schematically illustrates an approximation of the way in which the peak amplitude of the signals induced in y-sensor windings of the digitising system vary with the y-coordinate of the position of the stylus relative to the liquid crystal display;
  • Figure 5a illustrates the form of a sin x sensor winding of the digitising system which forms part of the-.personal digital assistant shown in Figure 1;
  • Figure 5b illustrates the form of a cos x sensor winding of the digitising system which forms part of the personal digital assistant shown in Figure 1;
  • Figure 5c illustrates the form of a sin y sensor winding of the digitising system which forms part of the personal digital assistant shown in Figure 1;
  • Figure 5d illustrates the form of a cos y sensor winding of the digitising system which forms part of the personal digital assistant shown in Figure 1;
  • Figure 6 is a plot illustrating the way in which the resonant frequency of the stylus changes with the gap between the stylus and the writing surface;
  • Figure 7a is an exploded perspective view of the resonant stylus shown in Figure 1;
  • Figure 7b is a cross-sectional view of the resonant stylus shown in Figure 1;
  • Figure 8a is a cross-sectional view of part of the resonant stylus shown in Figure 7b in an unclicked state, illustrating the positional relationship between a nib, a ferrite core and a coil forming part of the resonant stylus and showing magnetic field lies passing from the ferrite core around the coil in the unclicked state;
  • Figure 8b is a cross-sectional view of part of the resonant stylus shown in Figure 7b in a clicked state showing the positional relationship between the nib, ferrite core and coil of the resonant stylus and showing magnetic field lines passing from the ferrite core around the coil in the clicked state;
  • Figure 9 is a plot illustrating the percentage frequency change of the resonant frequency with gap between the ferrite rod and the split washer
  • Figure 10 is a diagrammatical view of test equipment used to test the resonant frequency of the stylus during manufacture to ensure that the clicked and unclicked resonant frequencies fall within required tolerances;
  • Figure 11 is a frequency plot illustrating a required unambiguous frequency detection range required of the positioning system and illustrating the range over which the resonant frequency of the stylus may vary between the clicked state and the unclicked state;
  • Figure 12 is a block diagram illustrating the functional modules forming part of a digital processing and signal generation unit forming part of the excitation and processing electronics shown in Figure 3a;
  • Figure 13 is a plot illustrating the way in which the electrical phase of the sensor signals varies with the difference in frequency between the resonant frequency of the stylus and the excitation frequency;
  • Figure 14a is a plot illustrating how an in-phase component (I) and a quadrature phase component (Q) vary with the difference between the resonant frequency of the stylus and the excitation frequency and also illustrating the way in which the amplitude of the sensed signals varies with the difference between these frequencies;
  • Figure 14b is a plot illustrating the same signals illustrated in Figure 14a except with a different number of excitation and reception cycles of the excitation- detection sequence;
  • Figure 15 is a plot showing how two amplitude estimates from different excitation frequencies vary with the resonant frequency of the stylus and illustrating two substantially linear plots obtained from these two amplitude measures ;
  • Figure 16 is a block diagram illustrating the components of a stylus frequency determining unit that is used in an embodiment which uses an amplitude method to estimate the resonant frequency of the stylus;
  • Figure 17a is a plot illustrating the way in which an in- phase component obtained from first and second excitation frequencies vary with the resonant frequency of the stylus and illustrating an offset which is obtained and which is removed by subtracting the two measurements;
  • Figure 17b is a plot for quadrature phase values corresponding to the in-phase values shown in Figure 17a;
  • Figure 18 is a block diagram illustrating the components of a stylus frequency determining unit that is used in an embodiment in which three excitation-detection sequences are used with different excitation frequencies and which uses offset elimination techniques to remove offsets within the measurements;
  • Figure 19a is a plot illustrating the way in which in- phase and quadrature phase components obtained from three excitation-detection sequences, vary with the resonant frequency of the stylus and the way in which., a phase estimate obtained from those in-phase and quadrature phase measurements varies with the resonant frequency of the stylus;
  • Figure 19b is a plot showing the way in which the phase value shown in Figure 19a varies with the resonant frequency of the stylus together with an amplitude function derived from the in-phase and quadrature phase measurements from the three excitation-detection sequences;
  • Figure 20 is a block diagram illustrating the components of a stylus frequency determining unit that is used in an embodiment in which an amplitude measure is used to resolve the phase ambiguity associated with a phase measure of the resonant frequency of the stylus;
  • Figure 21a is a plot illustrating the way in which two in-phase measurements and two quadrature phase measurements vary with the resonant frequency of the stylus
  • Figure 21b is a plot illustrating the way in which two phase measurements vary with the resonant frequency of the stylus and the way in which a combined phase measurement varies with the resonant frequency of the stylus;
  • Figure 22 is a schematic functional block diagram illustrating an alternative form of the excitation and processing electronics to that shown in Figure 3a in which different receive gate control signals are used for the processing channels for the x-direction sensor windings and the processing channels for the y-direction sensor windings;
  • Figure 23 is a block diagram illustrating the components of a stylus frequency determining unit that is used in a preferred embodiment
  • Figure 24a is a partial cross-sectional view illustrating an alternative arrangement of the stylus in an unclicked state
  • Figure 24b is a partial cross-sectional view of the stylus shown in Figure 14a in the clicked state
  • Figure 25a is a partial cross-sectional view of an alternative stylus in an unclicked state
  • Figure 25b is a partial cross-sectional view illustrating the stylus shown in Figure 15a in the clicked state
  • Figure 26 is a partial cross-sectional view of an alternative stylus whose resonant frequency can be varied at the time of manufacture using an adjustable pin;
  • Figure 27 is a partial cross-sectional view of another alternative stylus whose resonant frequency can be varied at the time of manufacture using a spacer having a selected thickness;
  • Figure 28 is a partial cross-sectional view of a stylus illustrating the way in which the resonant frequency of the stylus may be varied at the time of manufacture by adding an additional length of ferrite rod;
  • Figure 29 is a plot illustrating the way in which the resonator frequency changes with capacitor value with a fixed number of coils and with the number of coils being varied to maintain a relatively fixed resonator frequency;
  • Figure 30 is a plot illustrating the number of turns of conductor required on a coil forming part of the resonant stylus to maintain a given resonant frequency in dependence upon a measured value of the capacitance of a capacitor forming part of the resonant stylus;
  • Figure 31 is a perspective view showing a mobile telephone having a liquid crystal display and a digitising system under the display which is operable to sense the position of a resonant stylus relative to the display;
  • Figure 32a is a schematic view of a handheld personal digital assistant (PDA) which includes an x-y digitising system located behind the PDA's liquid crystal display which can sense the x-y position of a resonant stylus over a non-rectangular measurement area; and
  • PDA personal digital assistant
  • Figure 32b schematically illustrates the form of one of the sensor windings employed by the digitising system forming part of the PDA shown in Figure 32a.
  • Figure 1 shows a hand-held battery-powered personal digital assistant (PDA) 1 which employs an x-y digitising system (not shown) which is located beneath a liquid crystal display 3 of the PDA 1.
  • the x-y digitising system is operable to detect the presence and . x-y position of a resonant stylus 5 relative to the LCD 3.
  • the position signals output from the digitising system are used by the PDA 1 to control information that is displayed on the LCD 3 and to control the operating function of the PDA 1.
  • the PDA 1 also includes a number of push buttons beneath the LCD 3 including an on-off button 7 and a number of control buttons 9-1 to 9-4 which are used to control different functions of the PDA 1.
  • FIG. 2 shows a cross-sectional view on A-A of the PDA 1 shown in Figure 1.
  • the PDA 1 includes a liquid crystal display 3 which, in this embodiment, is between 1.5mm and 3mm thick.
  • this backlight layer 11 has a thickness of approximately 150 ⁇ m.
  • Beneath these layers, there is a 0.2mm thick sensor printed circuit board (PCB) 13 which forms part of the above-mentioned x-y digitising system.
  • This sensor PCB 13 carries the excitation winding and the sensor windings used for sending signals to and receiving signals from the resonant stylus 5.
  • Beneath the sensor PCB 13 there is a printed circuit board 15 which carries the electronics for controlling the functions of the PDA and the digitiser electronics for processing the signals received from and controlling the signals sent to the windings on the sensor PCB 13.
  • a grounded electrostatic screen 17 is provided between the sensor printed circuit board 13 and the electroluminescent backlight 11 in order to reduce noise from the liquid crystal display 3 and the backlight 11 from interfering with the x-y digitising system.
  • this electrostatic screen is formed from a continuous layer of carbon ink which is approximately lO ⁇ m thick and has a relatively high surface resistivity (e.g. > 1 ohm per square) so that it does not interfere with the magnetic sensing function.
  • the magnetic screen 21 is provided in order to reduce any disturbance which may be caused to the x-y digitising system by, for example, the electronics behind the sensor PCB 13. It also enhances the sensitivity of the x-y digitising system since it provides a permeable path for magnetic flux to pass behind the sensor windings on the sensor PCB 13.
  • an outer casing 23 which is made, in this embodiment, from plastic.
  • Figure 3a schematically illustrates a functional block diagram of the digitising system's processing electronics and Figure 3b illustrates some of the signals in the digitising system during an excitation and receive cycle.
  • Figure 3a also illustrates the way in which the excitation winding and the sensor windings interact with the resonant stylus 5.
  • Figure 3 schematically shows an excitation winding 29, two x- sensor windings 31 and 33 for sensing x position and two y-sensor windings 35 and 37 for sensing y position.
  • Each of these windings is formed by printed conductors on the sensor PCB 13.
  • the sensor windings 31, 33, 35 and 37 used in this embodiment are periodic and are in spatial phase quadrature relative to each other.
  • x-sensor winding 31 will be referred to as the sin x sensor winding
  • x-sensor 33 will be referred to as the cos x sensor winding
  • y-sensor winding 35 will be referred to as the sin y sensor winding
  • y-sensor winding 37 will be referred to as the cos y sensor winding.
  • these windings are operable, in use, to couple magnetically with a resonant circuit 41 (comprising a capacitor 43 and an inductor coil 45) in the resonant stylus 5.
  • an excitation current is applied to the excitation winding 29 through an excitation driver 51.
  • the excitation current comprises a sequence of positive and negative pulses having a fundamental frequency component (F 0 ) of approximately 100kHz, which is approximately the resonant frequency of the resonant circuit 41.
  • This excitation signal is generated by a variable frequency generator 53 which generates an appropriate excitation voltage which is applied to the excitation driver 51 through a switch 55.
  • the frequency of the excitation voltage generated by the generator 53 is set by an excitation/receive frequency control circuit 57 which forms part of a digital processing and signal generation unit 59.
  • the digitising system can be reconfigured to operate with a stylus having a different resonant frequency.
  • the excitation current flowing in the excitation winding 29 generates a corresponding electromagnetic field which magnetically couples, as indicated by the arrow 39-1, with the resonant circuit . 41 and causes it to resonate.
  • the excitation winding 29 is arranged to keep the coupling with the resonator as constant as possible with the x-y position of the stylus relative to the LCD 3.
  • the resonator 41 When the resonator 41 is resonating, it generates its own electromagnetic field which magnetically couples, as represented by the arrows 39-2, 39-3, 39-4 and 39-5, with the sensor windings 31, 33, 35 and 37 respectively.
  • the sensor windings 31, 33, 35 and 37 are designed so that the coupling between them and the resonant stylus varies with the x or y position of the stylus and so that there is minimum direct coupling between them and the excitation winding 29. Therefore, the signal received in the sensor windings should only vary with the magnetic coupling between the resonator 41 and the respective sensor winding. Consequently, by suitable processing of the signals received in the sensor windings, the x-y position of the resonator 41, and hence of the resonant stylus 5, can be determined relative to the sensor windings .
  • the excitation current is not continuously applied to the excitation winding 29. Instead, bursts of the excitation current are applied, with the application of the excitation bursts being controlled by opening and closing the switch 55. As shown in Figure 3a, this is controlled by an excitation gate controller 61 which forms part of the digital processing and signal generation unit 59.
  • the signals induced in the sensor windings are only detected between the bursts of the excitation current. This is achieved by controlling the positions of switches 63 and 65 with the receive gate controller 67 which forms part of the digital processing and signal generation unit 59.
  • This mode of operation is referred to as pulse echo and works because the resonator .41 continues to resonate after the burst of excitation current has ended. This mode of operation also minimises power consumption of the digitiser.
  • Figure 3b shows the excitation gate signal 30-1 applied to the switch 55; the excitation voltage 30-2 applied to the excitation winding 29; the receive gate signal 30-3 applied to the switches 63 and 65 and a typical voltage 30-4 induced in one of the sensor windings.
  • sixteen excitation cycles are applied to the excitation winding 29 which energises the resonator 41 in the stylus 5 which in turn induces a signal such as 30-4 in each of the sensor windings.
  • the four signals induced in the four sensor windings from the resonant circuit 41 can be approximated by: + ⁇ ] ( 3 )
  • E 37 Ae "/ ⁇ cos 2 ⁇ y cos[2 ⁇ E 0 t + ⁇ ] ( 4 )
  • A is a coupling coefficient which depends upon, among other things, the distance of the stylus 5 from the windings and the number of turns in the sensor windings; x is the x-position of the resonant stylus relative to the sensor windings; y is the y-position of the resonant stylus relative to the sensor windings; L x is...a spatial wavelength of the sensor windings in the x-direction and is typically slightly greater than the width of the board in the x-direction (and in this embodiment is 97mm); L y is a spatial wavelength of the sensor windings in the y- direction and is typically slighter greater than the width of the board in the y-direction (and in this embodiment is 87mm) ; e -t/t is the exponential decay of the resonator signal after the burst of excitation signal has ended, with ⁇ being a resonator constant which depends upon, among other things, the quality factor of the resonant circuit 41; and ⁇ is an electrical
  • the peak amplitude of the signals induced in the sensor windings vary as the sin or cos of either the x or y position.
  • Figures 4a and 4b illustrate the way in which the peak amplitude of the signal induced in sensor winding 31 and the way in which the signal induced in sensor winding 33 varies with the x-position of the resonant stylus relative to the sensor windings
  • Figure 4b shows the way in which the peak amplitude of the signals induced in sensor winding 35 and sensor winding 37 vary with the y-position of the resonant stylus relative to the sensor windings.
  • the pitch (L x ) of the windings in the x-direction is greater than the pitch (L y ) of the windings in the y-direction. This is because, in this embodiment, the measurement area is rectangular.
  • both the x-y position information of the resonant stylus 5 and the phase shift ⁇ can be determined from the signals induced in the sensor windings by suitable demodulation and processing.
  • this demodulation is achieved by mixing the received signals with the excitation voltage generated by the variable frequency generator 53 in the mixers 69-1 to 69-8.
  • an in-phase component 30-5 and a quadrature phase component 30-6 (shown in Figure 3b) of the excitation signal are mixed with the signal induced in each of the sensor windings.
  • This generates an in phase (I) component 30-7 and a quadrature phase (Q) component 30-8 of each of the demodulated signals.
  • the in phase components 30-7 of the demodulated signals from all the sensor windings are used to determine the position information and the in phase and quadrature phase components of the demodulated signals are used to determine the electrical phase shift (i.e. ⁇ ).
  • the output from these mixers 69 are input to a respective integrator 71-1 to 71-8 which, after being reset, integrate the outputs from the mixers over a time period which is a multiple of 1/F 0 (in order to remove the effect of the time varying components output by the mixer) .
  • the integration time is controlled by using the receive gate signal 30-3 (which in the illustration allows for the integration to be performed over sixteen excitation periods or cycles).
  • the outputs from the integrators 71 are input to an analogue-to-digital converter 73 which converts the outputs into digital values which are input to the A to D interface unit 75 in the digital processing and signal generation unit 59.
  • the digital processing and signal generation unit 59 then performs an arc tangent function (atan 2 ) on the ratio of the sin_x__I signal and the cos_x_I signal to determine the x-position of the resonant stylus 5 and similarly performs an arc tangent function on the ratio of the sin_y_I signal and the cos_y_I to determine the y-position of the resonant stylus 5.
  • the digital processing and signal generation unit 59 also calculates an arc tangent function on the ratio of the quadrature phase component to the in phase component of the signals from the same sensor windings, in order to determine the electrical phase angle ⁇ .
  • the digital processing and signal generation unit 59 determines the electrical phase angle ⁇ using a weighted combination of the in phase and quadrature phase signals from both the sin and cos windings, where the weighting used varies in dependence upon the determined x and y position of the stylus 5.
  • the processing electronics uses this electrical phase angle measurement to determine if the tip of the stylus 5 has been brought down into contact with the writing surface of the PDA.l. The way in which this is achieved will be described in more detail later.
  • the digital processing and signal generation unit 59 After the digital processing and signal generation unit 59 has determined the current x-y position of the resonant stylus 5 and determined whether or not the stylus 5 has been brought into contact with the LCD 3, it outputs this information to the PDA electronics through the interface unit 77. This information is then used by the PDA electronics to control information displayed on the LCD 3 and the PDA's mode of operation.
  • the digital processing and signal generation unit 59 is operable to perform the above calculations approximately 100 times per second when the stylus is in the vicinity of the PDA. However, when the system detects that the stylus is not present, it initially enters a standby state in which the above excitation and processing is performed approximately 20 times per second. After a predetermined length of time in this standby state, the system enters a sleep state in which the above calculations are performed approximately 2 times per second. Once the presence of the stylus is detected again, the processing resumes at the 100 times per second rate.
  • the excitation winding 29 used in this embodiment is formed by two turns of rectangular conductor on each of the four layers of the sensor PCB 13.
  • the conductors formed on the four layers are shown as full lines in Figures 5a to 5d.
  • the two turns of the excitation winding 29 on each layer are wound around the outside of the conductors forming the sensor windings at the perimeter of the sensor PCB 13 and are connected in series with each other at through holes or vias, some of which are labelled 97.
  • Figure 5a also shows the conductors which form the sin x sensor winding 31.
  • the majority of the conductors forming the sin x sensor winding 31 are provided in a single layer which in this embodiment is the layer closest to the magnetic screen 21.
  • the conductors of the sin x sensor winding 31 on this lower layer of the sensor PCB 13 are shown in full lines, whereas those conductors of the sin x sensor winding 31 which are formed on other layers of the sensor PCB 13 are shown as dashed lines.
  • the conductor tracks on the different layers are connected together at via holes, some of which are labelled 97.
  • the conductor tracks of the sin x sensor winding 31 are connected to form a number of multi-turn loops which are arranged in succession along the x-direction.
  • the loops of the sin x sensor winding 31 are connected in series and arranged so that the magnetic coupling between the resonant stylus 5 and the winding 31 varies in an approximate sinusoidal manner in the x-direction across the sensor PCB 13.
  • the loops of the sensor winding 31 are also arranged so that there is substantially no variation in the magnetic coupling between the resonant stylus 5 and the sensor winding 31 if the resonant stylus 5 is moved across the sensor PCB 13 in the y-direction. In this way, the signal induced in the sensor winding 31 by the resonant stylus 5 will have a peak amplitude which approximately varies as the sine of the x-position of the stylus 5 relative to the sensor winding 31.
  • Figure 5b shows some of the conductors which form part of the excitation winding 29 and the conductors which form the cos x sensor winding 33.
  • the majority of the conductors forming the cos x sensor winding 33 are provided in a single layer of the PCB 13, which in this embodiment is the second closest layer to the magnetic screen 21.
  • the conductors of the cos x sensor winding 33 on this layer of the sensor PCB 13 are shown in full lines, whereas those conductors of the cos x sensor winding 33 which are formed on other layers are shown as dashed lines.
  • the conductor tracks on the different layers are connected together at via holes, some of which are labelled 97.
  • the conductor tracks of the cos x sensor winding 33 are also connected to form a number of multi-turn loops which are arranged in succession along the x-direction.
  • the loops are also connected in series so that as the resonant stylus 5 moves across the sensor winding 33 along the x-direction, the magnetic coupling between the resonant stylus 5 and the sensor winding 33 will vary in a substantially sinusoidal manner with the x-position of the resonant stylus 5.
  • the loops of the cos x sensor winding 33 are also arranged relative to the loops of the sin x sensor winding 31 so that this sinusoidal variation is substantially 90° out of phase with the corresponding sinusoidal variation associated with the sin x sensor winding 31.
  • the loops of the cos x sensor winding are arranged so that the magnetic coupling between the resonant stylus 5 and the sensor winding 33 does not vary with the y-position of the stylus 5 relative to the sensor winding 33. Therefore, when the resonant stylus 5 is resonating, it will induce a signal in the sensor winding 33 which has a peak amplitude which approximately varies as the cosine of the x-position of the stylus 5 relative to the sensor winding 33.
  • Figures 5c and 5d show the other conductors forming the excitation winding 29 and the conductors which form the sin y sensor winding 35 and the cos y sensor winding 37. As shown in these figures, these sensor windings 35 and 37 are similar to the sin x and cos x sensor windings 31 and 33, except they are rotated through 90°.
  • Figure 5d also shows the connection pads which are provided for connecting the ends of the sensor windings to the processing electronics.
  • the ends of the sin x sensor winding 31 are connected to connection pads 105 and 107; the ends of the cos x sensor winding 33 are connected to connection pads 109 and 111; the ends of the sin y sensor winding 35 are connected to connection pads 107 and 113; and the ends of the cos y sensor winding 37 is connected to connection pads 109 and 115.
  • the sin y sensor winding 35 shares the connection pad 107 with the sin x sensor winding 31 and the cos y sensor winding 37 shares the connection pad 111 with the cos x sensor winding.
  • Figure 5d also shows the connection pads 101 and 103 which connect the excitation winding 29 to the excitation driver 51 (shown in Figure 3a).
  • Figure 5d also shows a connection pad 117 which is for connection to a ground terminal (not shown) of the PDA device. As shown in Figure 5d, this ground connection pad 117 is connected to a castellated conductor region 119 which provides the ground connection for the electrostatic screen 17.
  • the conductor region 119 is castellated in order to reduce eddy currents from being generated therein.
  • the grounded conductor region 119 is also connected (through via holes 97) to a grounding pad 121, formed on the lower layer of the PCB 13 shown in Figure 5a, for grounding the magnetic screen 21. 5243
  • the design of the sensor windings is one of the most critical aspects of the digitiser.
  • the design involves, for a given area of printed circuit board, maximising the digitising area and the accuracy of and the signal levels from the sensor windings.
  • the conductor tracks of the x-position sensor windings 31 and 33 which extend across the sensor board in the y-direction will be referred to as the transverse conductors and those which extend in the x-direction will be referred to as the connecting conductors.
  • the y-position sensor windings 35 and 37 which extend across the sensor board in the x- direction will be referred to as the transverse conductors and those which extend in the y-direction will be referred to as the connecting conductors .
  • transverse conductors 31-t, 33-t, 35-t and 37-t and some of the connecting conductors are illustrated by reference numerals 31-c, 33-c, 35-c and 37-c in Figures 5a to 5d.
  • the most striking feature of most of the transverse sensing conductors is their irregular form with multiple bends or kinks as they extend from one side of the sensor board 13 to the other.
  • these transverse conductors are formed by substantially straight parallel lines.
  • the applicant has found that the use of such irregular shaped transverse conductors can surprisingly result in more accurate position sensing by the digitising electronics.
  • These irregular transverse conductors can provide accurate position sensing because positional errors caused by irregularities or bends of the transverse conductors of the sin winding can be compensated by complementary irregularities or bends in the transverse conductors of the cosine winding. These errors then cancel with each other when the arc tangent function is calculated by the digitiser electronics, thereby giving a more accurate position measurement.
  • each of the other sensor windings 33, 35 and 37 includes such inwardly extending corner areas, labelled 33-1 to 33-4, 35-1 to 35-4 and 37-1 to 37-4.
  • these corner portions of the y-direction sensor windings extend inwardly along the y-direction towards a central portion of the y-direction sensor windings.
  • the effect of these "bulging” or “flared” corners is to increase the mutual inductance between the sensor coils and the resonant stylus 5 near the sensor board corners which therefore increases the minimum signal EMFs induced therein.
  • the areas with increased coupling are restricted to the perimeter of the sensor board 13, the coupling to unwanted noise sources is not increased unnecessarily.
  • the cos x sensor winding 33 and the cos y sensor winding 37 include additional design features which help to boost the signal levels obtained from the sensor windings when the stylus 5 is at the perimeter of the sensor board 13.
  • additional loops of conductor generally referenced 37-5 and 37-6 are provided at the left and right-hand edge of the sensor board 13. As a result of these additional loops, the signal levels from the resonant stylus 5 are boosted at these left and right-hand edges compared with the signal levels obtained when the stylus 5 is in the centre of the PCB 13.
  • the additional loops 33-5 and 33-6 in the cos x sensor winding 33 are provided at the top and bottom of the PCB 13 and are connected together across the middle of the PCB 13.
  • the conductors which form this connection are close together in the x- direction so that the increase in coupling is minimised except near the top and bottom of the PCB 13.
  • This reduction in the coupling in the central region can alternatively be viewed as an increase in the relative coupling near the top and bottom edge of the sensor PCB 13.
  • the inventors have found that with these additional design features of the sensor windings leads to increased accuracy of the x-y digitiser.
  • the stylus 5 of the present embodiment overcomes a number of problems with previous styluses which have been proposed and in particular the stylus proposed in WO 00/33244 described above.
  • the stylus 5 is also designed to be sufficiently compact for space-critical applications such as the hand-held PDA 1 of the present embodiment.
  • the resonant stylus 5 in this embodiment comprises a resonant circuit 41 which includes an inductor coil 45 and a capacitor 43.
  • the resonant stylus 5 is also designed so that the resonant frequency of the resonant circuit 41 changes when the tip of the stylus 5 is brought down into contact with the writing surface of the digitising system.
  • Figure 6 shows a plot 10 illustrating the way in which the resonant frequency of the stylus 5 used in this embodiment changes with the gap between the stylus 5 and the writing surface of the PDA 1.
  • the resonant frequency of the stylus 5 decreases (due to the detuning effect of the magnetic screen 21) to a value of f uo at the point where the nib 159 of the stylus 5 touches the writing surface of the PDA 1.
  • the nib is pushed back into the stylus body into its clicked state, at which point the resonant frequency of the stylus has increased to f c .
  • the processing electronics can determine whether or not the stylus 5 is in its clicked state or unclicked state.
  • the change in resonant frequency between the unclicked and clicked states must be greater than the change in frequency caused by the detuning effect of the magnetic screen 21. Therefore, in this embodiment, the stylus 5 is designed to provide a change in resonant frequency of approximately 10% between the unclicked and clicked states. The stylus 5 is also designed so that this change in frequency can be achieved while keeping to a minimum the distance over which the nib of the stylus 5 must travel between the clicked and unclicked states.
  • Figure 7a shows an exploded view of the components of the resonant stylus 5 used in this embodiment.
  • the stylus 5 comprises a hollow front body portion 152 and a hollow rear body portion 154 which house: the resonant circuit 41 comprising the inductor coil 45 and the capacitor 43; a 2mm diameter ferrite rod 153; a plastic sleeve 155 having an inner diameter of 2.1mm and an outer diameter of 2.2mm; a split washer 157; a nib 159; and a spring 163.
  • the coil 45 is manufactured from self- bonding enamelled copper wire for low-cost by eliminating a coil former. The ends of the coil 45 are welded to the side of a surface mount capacitor 43 to form the resonant circuit 41.
  • the plastic sleeve 155 having a thin wall section (of approximately 50 microns) made from spirally wound and bonded plastic sheet fits inside the coil 45 and acts as a bearing surface for the ferrite rod 153 and prevents the ferrite rod 153 from rubbing against the capacitor 43 during use.
  • the plastic sleeve 155 has a much thinner cross-section than can be achieved with an injection-moulded component, thereby enabling higher resonator Q-factor and hence lower system power consumption.
  • the pen is manufactured as follows. The plastic sleeve 155 is pressed into the coil 45 and glued in place. This assembly is then placed into a jig (not shown) where the capacitor 43 is offered up and held in position.
  • the wire ends of the coil 45 are positioned either side of the capacitor 43 and are welded in place by a welding head (not shown).
  • the nib 159 component is dropped into the front body portion 152 , followed by the split washer 157 and the coil assembly.
  • the ferrite rod 153 is then dropped into the plastic sleeve 155.
  • the spring 163 is then dropped into the rear body portion 154 and the forward body portion 152 and the rear body portion 154 are connected together and glued in position.
  • the front body portion 152 and the rear body portion 154 are forced tightly together so that the neck portion 166 forces the coil 45 against the split washer 157 and a first shoulder 167 of the front body portion 152.
  • the coil 45 and the split washer 157 are fixed in position with respect to the stylus body, with, in this embodiment, the coil 45 being positioned towards a front face 153a of the ferrite rod 153.
  • the neck portion 166 of the rear body portion 154 includes a slot for receiving the capacitor 43 when the front and rear body portions are pushed together. This avoids the need for long coil leads which would be required were the capacitor 43 to be mounted behind the spring 163, and avoids increased assembly complexity and cost.
  • Figure 7b shows the assembled stylus 5 in cross-section.
  • the nib 159 and the ferrite rod 153 are slidably mounted within the stylus body and spring-biased (by spring 163) towards the front end 161 of the front body portion 152.
  • the movement of the ferrite rod 153 in this forward direction is, however, limited by the abutment of a front face 160a (shown in Figure 7a) of an enlarged head 160 of the nib 159 with a second shoulder 168 (shown in Figure 7b) of the front body portion 152.
  • the ferrite rod 153 can, therefore, only move a predetermined distance (d 0 ), referred to hereinafter as the click- distance, when pressure is applied to the end of the nib 159.
  • the stylus 5 is designed so that the click distance (d 0 ) is 0.35mm.
  • This movement of the front face 153a of the ferrite rod 153 from the front face 45a of the coil 45 causes a decrease in the inductance of the coil 45 due to the reduced coupling between the ferrite rod 153 and the coil 45, which in turn gives rise to an increase in the resonant frequency of the resonant circuit 41.
  • Figures 8a and 8b are a partial cross-sectional views of the assembled stylus 5 showing in more detail the relative positions of the ferrite rod 153, the coil 45, the split washer 157 and the nib 159 in these "unclicked” and “clicked” states respectively, and illustrating magnetic field lines 180 passing from the end of the ferrite rod 153 around the coil 45.
  • the ferrite rod 153 is close to the split washer 157, which in this embodiment, is made of Vitrovac 6018, which is a high magnetic permeability material. Therefore, a relatively strong local magnetic field is established with resonating current in the coil 45 as illustrated by the tightly spaced magnetic field lines 180a in Figure 8a.
  • the radial extent of the locally strong magnetic field 180 is approximately from the inner diameter of the split washer 157 to between the inner and outer radius of the coil 45.
  • the reason for the locally strong magnetic field 180a is that both the ferrite rod 153 and the washer 157 have high magnetic permeability, and the distance between the ferrite rod 153 and the split washer 157 is relatively small compared to the radial extent of the locally strong magnetic field. Consequently, magnetic field couples easily from the ferrite rod 153 into the split washer 157, rather than passing out through the side of the coil 45.
  • Figure 9 is a plot illustrating how the resonant frequency of the resonator 41 changes with the gap between the ferrite rod 153 and the end face 45a of the coil 45 with the split washer (plot 8-1) and without the split washer 157 (plot 8-2).
  • the stylus 5 with the split washer 157 provides approximately an 8% change in the resonant frequency of the resonator 41 between the unclicked and clicked states.
  • a change in resonant frequency of about 3.5% is achieved. Therefore, the use of the split washer 157 allows a greater change in resonant frequency between the clicked and unclicked states for a given click distance. As discussed in the introduction of this application, this is important where 02 05243
  • a critical component of the manufacturing variability of the stylus 5 is the position of the ferrite rod 153 relative to the end face 45a of the coil 45 and the split washer 157.
  • the position is set by only two plastic dimensions - the first is the distance between rear face 159a of the nib 159 and front face 160a of the nib's head 160; and the second is the distance between the first shoulder 167 and the second shoulder 168 of the front body portion 152. Since these distances are relatively small (a few millimetres) and close together, it is relatively straightforward to maintain tight control of these distances and therefore tight control of the unclicked frequency of the stylus 5.
  • the position of the ferrite rod 153 relative to the end face 45a of the coil 45 and the split washer 157 is defined by the distance between rear face 159a of the nib 159 and rear face 160b of the nib's head 160.
  • the manufacturing cost of the stylus 5 is relatively low.
  • the plastic parts controlling the relative position of the ferrite rod 153, the coil 45 and the split washer 157 are subject to thermal expansion, because these critical dimensions are relatively small and close together, the position changes little with temperature.
  • the thickness of the split washer 157 also has an effect on the relative position of the ferrite rod 153, the coil 45 and the washer 157, but that thickness is well controlled because the washer material may either be manufactured from a punched sheet of metal formed in a rolling process or by a suitable etching process.
  • a sheet of the material may be covered with a photoresist, preferably on both sides, and then the resist exposed to ultraviolet light through a mask patterned with the required shape. The sheet is then etched in chemical solution leaving the washers, usually held by a spike of metal to the original sheet. The washers are then cut from the sheet and assembled into styluses.
  • An advantage of etching is that there is no mechanical stressing so that there is no loss in. magnetic permeability that would otherwise reduce frequency shift and introduce variability.
  • the resonant frequency of each stylus 5 is tested before the front body portion 152 is glued together with the rear body portion 154.
  • This testing is performed by the testing apparatus 200 schematically illustrated in Figure 10.
  • the testing apparatus 200 includes a pulsed current source 201 which applies a pulse of excitation current to a coil 203 which is magnetically coupled to the coil 45 in the stylus 5 which causes the resonant circuit 41 to resonate.
  • the current from the pulsed current source 201 is then stopped and the resonator 41 continues to resonate and this resonating signal induces an EMF in a second coil 205 wound around the stylus 5.
  • This induced EMF is then passed to a signal detector, processor and display unit 207 which measures the frequency of the ring-down signal, for example by performing a Fourier analysis of the sampled waveform.
  • the signal detector, processor and display unit 207 also controls a nib actuator 209 which applies pressure to the nib 159 forcing the stylus 5 into its clicked state.
  • the same excitation and measurement process is then carried out to determine the resonant frequency of the stylus 5 in the clicked state.
  • the signal detector, processor and display unit 207 then compares the unclicked resonant frequency and the clicked resonant frequency with predefined manufacturing limits and the stylus assembly is rejected if the measured values fall outside those limits.
  • the rear body portion 154 is glued to the front body portion 152.
  • the advantage of testing the partially assembled stylus 5 is that if the measured clicked and unclicked resonant frequencies fall outside the manufacturing limits, then the failure is identified earlier in the manufacturing process and hence there is less wastage.
  • each stylus 5 is designed so that their clicked and unclicked resonant frequencies lie within a "free space clicked resonant frequency band" and a "free space unclicked resonant frequency band", respectively. These are shown in Figure 11 as the unclicked band B 2 between frequency fj and f 2 and the clicked band B 2 between frequency f 6 and f 7 .
  • the system is designed, however, to be able to detect pen frequencies over a much larger frequency range (DR) extending from frequency f 0 to f 8 .
  • DR frequency range
  • margins are illustrated by the bands M x d extending from frequency f 0 to fi; margin M] 1 extending from frequency f 2 to f 3 ; margin M 2 d extending from frequency f 5 to f 6 ; and margin M 2 i extending from frequency f 7 to f 8 .
  • An overall unclicked frequency band B x extending from frequency f 0 . to frequency f 3 and a clicked frequency band B x extending from frequency f 5 to frequency f 8 are therefore defined.
  • a frequency spacing is also provided (labelled PE and extending from frequency f 3 to f 5 ) to account for phase detection inaccuracy in the electronics which results in uncertainty for frequencies close to the threshold frequency (f 4 ) used to determine the click state of the stylus 5.
  • the digital values output by the analogue-to-digital converter 73 are passed via the A to D interface 75 to a buffer 251.
  • a buffer 251 At the end of a pulse echo excitation/receive cycle, eight digital values will be stored in the buffer 251 representing the in-phase and quadrature phase signal levels generated for each of the four sensor windings.
  • a control unit 253 is provided for reading out these digital values and for passing them to the appropriate processing modules for processing.
  • the control unit 253 initially passes the digital signal values to a signal level comparator 255 which compares the signal levels with a threshold value. If all of the signal levels are below the threshold level, then this indicates that the stylus 5 is not in the vicinity of the PDA 1 and therefore, no further processing is required.
  • the control unit 253 passes the in-phase signal components to a position processor 257 which calculates the above-described arc tangent functions using the in-phase components to determine the x-y position of the stylus 5 relative to the sensor board 13.
  • the control unit 253 also passes the in-phase and quadrature phase components to a stylus frequency determining unit 259 which, as discussed above, performs the above mentioned arc tangent function on the in-phase and quadrature phase components of the signals from the same sensor winding, to generate a measure of the electrical phase ( ⁇ ) of the received signal.
  • This electrical phase can then be mapped to a difference in frequency between the resonant frequency of the stylus 5 and the fundamental frequency F 0 of the excitation signal applied to the excitation winding 29.
  • the relationship relating this phase measurement to the frequency difference is cyclic in nature and can therefore only provide a unique one-to-one relationship between the measured phase and the resonant frequency for a limited range of frequency differences. Further, this range of frequency differences depends, among other 02 05243
  • plot 300-1 shows the measured electrical phase ( ⁇ ) plotted on the y-axis against the ratio of the stylus 5 resonant frequency to the excitation frequency as a percentage.
  • this plot 300-1 is linear only for the ratio between the stylus frequency and the excitation frequency varying (as a percentage) between 98.2 and 101.7. Outside this range, the plot 300-1 repeats in a non-linear and cyclic manner.
  • the system can therefore only unambiguously determine the resonant frequency of the stylus 5 if it is within a range of 1.75% of the excitation frequency on either side of the excitation frequency. This is sufficient for the type of stylus described in the applicant's earlier International application WO 00/33244, but not for the stylus 5 described above which is designed so that the resonant frequency changes by approximately 8% between its clicked and unclicked states.
  • the resonant frequency of the stylus 5 can be determined unambiguously provided it is between 91% and 108% of the excitation frequency. This corresponds to a range of approximately 17%, which is sufficient for the stylus 5 used in this embodiment.
  • a further problem with using a small number of transmission cycles and reception cycles is that more energy is spread over the entire frequency band of operation, which reduces the power efficiency of the device as a whole.
  • the issue of low power efficiency is described in detail in the applicant's earlier International application WO 01/29759. As described in this earlier International application, such low power efficiency systems are undesirable in hand-held battery- powered devices such as the PDA 1 of the present embodiment.
  • a pulse-echo excitation/reception cycle is performed with N ⁇ x and N RX set to 3, with the fundamental frequency F 0 of the excitation signal being in the middle of the required frequency range (i.e. approximately at the decision frequency f 4 shown in
  • This first stage measurement will provide an electrical phase measurement that is unambiguous over the required frequency range (DR) .
  • the stylus frequency determined by the stylus frequency determining unit 259 is output to a stylus state determining unit 261. If the determined stylus frequency is far enough from the decision frequency (f 4 ) then, with all errors accounted for, it is possible to determine the click state of the stylus 5. If the determined frequency is not far enough from the decision frequency (f 4 ), then the stylus state remains uncertain.
  • This closeness between the excitation frequency and the stylus resonant frequency allows for the greater number of transmission cycles and reception cycles to : ,be used, thereby allowing for more accurate position and phase measurements to be obtained and allowing for a more power efficient measurement cycle.
  • the excitation and receive control unit 57 receives the approximate resonant frequency of the stylus 5 determined by the stylus frequency determining unit 259 from the signals of the first stage measurement cycle. It then outputs a control signal to the variable frequency generator 53, setting the fundamental frequency (F 0 ) of the excitation and mixing signals to be generated.
  • the data from this second more accurate measurement cycle is then passed to the position processor 257 and the stylus frequency determining unit 259 as before, where more accurate estimates of the x-y position of the stylus 5 relative to the sensor board 13 and the resonant frequency of the stylus 5 are determined.
  • This more accurate measurement of the resonant frequency of the stylus 5 is then passed to the stylus state determining unit 261 which compares the measured frequency with the decision frequency f 4 to determine if the stylus 5 is in its clicked state or its unclicked state. This determination together with the accurate x-y position measurement is then passed to the PDA electronics via the interface 77.
  • the above processing provides a number of advantages. These include: i) power consumption of the system may be reduced by cancelling the second stage measurement if the first stage measurement does not detect the presence of the stylus 5; ii) the first measurement stage can be optimised to minimise power consumption since accurate detection of position and resonant frequency of the stylus 5 is performed in the second measurement stage; iii) in the second measurement stage, the excitation frequency transmitted may be at one of a set fixed number of frequencies spread between f 0 and f 8
  • the processing electronics can increase the power or increase the sensitivity of the detection circuits for the second measurement cycle (this is an advantage over the prior art systems where power level and sensitivity settings are set for worst case conditions , resulting in power consumption that is higher than is actually required on average).
  • the resonant frequency of the stylus 5 was determined from the electrical phase ( ⁇ ) of the sensed signals. This is possible because the electrical phase of the sensed signals varies linearly (over a limited range) with the difference between the excitation frequency and the resonant frequency of the stylus 5. Therefore, since the excitation frequency is known to the processing electronics, measuring the electrical phase ( ⁇ ) of the sensed signals allows the estimation of the resonant frequency of the stylus 5.
  • the above method of calculating the resonant frequency of the stylus 5 will be referred to as the "phase method", since it relies on the calculation of the electrical phase ( ⁇ ).
  • FIG. 14a is a plot illustrating how an in-phase component (I) and a quadrature phase component (Q) (output from the integrators 71) vary with the difference between the resonant frequency of the stylus 5 and the excitation frequency.
  • Figure 14a also includes a plot (labelled A e ) which represents how the amplitude of the sensed signals varies with the difference between the resonant frequency of the stylus 5 and the excitation frequency.
  • a e a plot which represents how the amplitude of the sensed signals varies with the difference between the resonant frequency of the stylus 5 and the excitation frequency.
  • the amplitude plot A e is a non-linear parabolic type plot which peaks when the stylus frequency matches that of the excitation frequency.
  • the shape of the amplitude plot A e also depends on the number of transmission and reception cycles (i.e. N ⁇ x and N RX ) in the excitation-detection sequence.
  • As shown in Figure 14b with more transmission and reception cycles, greater signal levels are generated and the amplitude plot A e varies more significantly with the difference between the resonant frequency of the stylus 5 and the excitation frequency.
  • the stylus frequency determining unit 259 calculates an estimate of the amplitude (Ax) from:
  • the digital processing and signal generation unit 59 performs two excitation-detection sequences at two different excitation frequencies and then compares the amplitude values determined for each excitation-detection sequence.
  • the digital processing and signal generation unit 59 performs a first excitation-detection sequence with the fundamental frequency of the excitation signal being at frequency f x (which in this embodiment is 100.5 kHz) and then performs a second excitation-detection sequence with the fundamental frequency of the excitation signal being at a frequency f 2 (which in this embodiment is 106 kHz).
  • the stylus frequency determining unit 259 calculates an estimate of the amplitude for each of the two excitation- detection sequences .
  • Figure 15 is a plot showing how the two amplitude estimates (A e (F ⁇ ) and A e (F 2 )) vary with the resonant frequency of the stylus 5.
  • Figure 15 also shows a plot of the following two functions:
  • m,, m 2 , c x and c 2 are system constants that are determined in advance from an appropriate calibration routine from the straight line that best matches the g and g 2 plots shown in Figure 15.
  • the above technique for estimating the resonant frequency of the stylus 5 will be referred to as the "amplitude method".
  • One important advantage of the amplitude method is that it allows for the unambiguous estimation of the resonant frequency of the stylus over a much broader range of frequency differences than using the phase method used in the first embodiment.
  • the two functions gi and g 2 are approximately linear for a resonant frequency of the stylus between 95 kHz and 111 kHz.
  • this range of unambiguous detection can only be achieved by significantly reducing the number of transmission and reception cycles (N ⁇ x and N RX ) which, as discussed above, has implications for signal levels and power efficiency.
  • Figure 16 is a block diagram illustrating in more detail the components of the stylus frequency determining unit 259 that would be used in an embodiment which used the above-described amplitude method to estimate the resonant frequency of the stylus 5.
  • the stylus frequency determining unit 259 used in this embodiment includes a signal amplitude determining unit 401 which receives the in-phase and quadrature phase signal levels (I and Q) for each of the two excitation-detection sequences, and calculates an amplitude measure for each using equation (9) above.
  • the signal amplitude determining unit 401 receives the in-phase and quadrature phase signal levels from the first excitation- detection sequence and stores the resulting amplitude value (A e (Fi)) in a buffer 403 until it receives the in- phase and quadrature phase signal levels from the second excitation-detection sequence.
  • the amplitude measure (A e (F 2 )) for the second excitation-detection sequence has been determined and stored in the buffer 403
  • the two amplitude measures are then passed to a logarithm determining unit 405 which performs the logarithm calculation given above in equation (10).
  • the value output by the logarithm determining unit 405 is then passed to a transformation unit 407 which estimates the resonant frequency of the stylus 5 using the linear transformation defined in equation (12) above.
  • the stylus frequency determining unit 259 then outputs the estimated frequency to the stylus state determining unit 261 as before.
  • amplitude measures from two excitation-detection sequences which use different excitation frequencies were used to estimate the resonant frequency of the stylus 5.
  • the use of two frequencies may not provide sufficient accuracy in the measurement.
  • This problem may be overcome by using amplitude measures from more than two excitation- detection sequences (each with a different excitation frequency) and then by using the two amplitude measurements that yield the highest amplitudes or by using a combination of all of the measurements.
  • N excitation frequencies Fi, F 2 ... F N with calculated amplitudes ⁇ F ⁇ , A e (F 2 ) ... A e (F n )
  • the following function may be employed instead of the logarithmic or arc-tangent function defined in equations (10) and (11) above:
  • the following function may be employed which generates an estimate based on the assumption that the amplitude measures vary approximately in a parabolic manner with the resonant frequency of the stylus 5 :
  • a e (F.)-A e (F 3 ) g 4 (A e ) . ° K l ⁇ , V , , (15) Q ) 2(A e (F 1 )-2A e (F 2 ) + A e (F 3 ))
  • the processing electronics illustrated in Figure 3a typically introduce an offset error into the signal levels applied to the analogue-to-digital converter 73.
  • the sin x in-phase and quadrature phase components may be represented as: 2 ⁇ x
  • SXI off and SXQ off are offsets which, in general, are a function of the particular mixer and integrator channel on a particular device, supply voltage, temperature and time. They may also depend on other factors specific to the circuit implementation, such as the integration time of the integrators. In some cases, these offsets may be sufficiently large to interfere with the estimation of the resonant frequency of the stylus 5. In this case, a calibration step could be employed in which these offsets are measured and then subtracted from the appropriate signal level.
  • An alternative approach is to base each measurement on two excitation-detection sequences, having the same excitation frequency but with inverted excitation voltage so that the amplitude term A ⁇ is inverted.
  • the offsets can then be removed by subtracting the respective measurements generated by the same channel from the two excitation-detection sequences .
  • the resonant frequency of the stylus 5 can then be estimated using either the phase method or the amplitude method from the offset compensated in-phase and quadrature phase components.
  • the problem with the above offset compensation technique is that it doubles the number of excitation-detection sequences that are required and this may limit the number of times the digital processing and signal generation unit 59 can determine the position of the stylus 5, and hence the ability of the system to respond to the user moving the stylus 5 across the sensor board.
  • the mixing signals applied to the sensed signals may be inverted.
  • inverting the mixing signals may not be preferred since the offset from each channel may be a function of the mixer waveform, such that an inverted waveform may yield a slightly different offset and hence imperfect cancellation of the offsets.
  • the offsets may be eliminated by combining the channel outputs from different excitation-detection sequences in which different excitation frequencies are used.
  • Figure 17a shows a plot I(Fi) which shows how the output of one of the in-phase channels varies with the resonant frequency of the stylus 5 with an excitation frequency of 100.5 kHz.
  • Figure 17a also shows a second plot I(F 2 ) showing how the same in-phase measurement from the same channel varies with the resonant frequency of the stylus 5 when an excitation frequency of 106 kHz is used.
  • Io ff approximately 50 units of offset (Io ff ) are included within these measurements.
  • Figure 17a also shows a plot I(F 2 -F ⁇ ) that it obtained by subtracting plot I(F X ) from plot I(F 2 ). As shown from this combined plot, the offset is cancelled while the signal remains. Further, in this illustration, the excitation frequencies have been chosen so that the plots I(F X ) and I(F 2 ) are approximately 180° out of phase with each other, so that their subtraction yields a larger wanted signal than either I(Fi) or I(F 2 ) alone. Whilst this is not necessary, it is preferred since it results in a measurement with good signal-to-noise.
  • Figure 17b shows the corresponding plots for the corresponding quadrature phase channel measurements Q(F ⁇ ) / Q(F z ) and the combined plot Q(F 2 -F! obtained from their subtraction. As shown, the offset from the quadrature phase channel is also cancelled, while maximising the wanted signal components, in a similar way as for the in-phase channel.
  • the resonant frequency of the stylus 5 may then be estimated from I(F 2 -F X ) and Q(F 2 -Fi) using the above- described phase method.
  • This approach has the advantage of removing errors caused by offsets and the use of two excitation frequencies broadens the frequency range over which useful signal level is obtained.
  • a further excitation-detection sequence may be employed at a third excitation frequency yielding I(F 3 ) and Q(F 3 ).
  • the measurements from the second and third excitation- detection sequence (or from the first and third excitation-detection sequence) may then be combined in the same way as the measurements from the first and second excitation-detection sequences to yield I(F 3 -F 2 ) and Q(F 3 -F 2 ).
  • a third excitation frequency of 111.5 kHz is preferred as this results in in-phase and quadrature phase components that are approximately 180° out of phase with the corresponding components of the second excitation-detection sequence, thereby resulting in maximum wanted signal levels after the combination.
  • the in-phase and quadrature phase measurements from the first and second excitation- detection sequences may then be combined to generate an amplitude measure A e (F 2 -F],) and similarly the in-phase and quadrature phase measurements from the second and third excitation-detection sequence may be combined to give an amplitude measure A e (F 3 -F 2 ).
  • the above-described amplitude method may then be used to estimate the resonant frequency of the stylus 5 using these two amplitude measures. For example by taking the logarithm of the ratio of these amplitude measures as follows: and then by applying the resulting measure g(F 3 ,F 2 ,F ! ) through an appropriate transformation equation similar to equation (12) above.
  • the resulting frequency estimate is therefore based on only three excitation-detection sequences, unlike the approach outlined earlier which used excitation signal inversion that requires four excitation-detection sequences.
  • This approach therefore takes less time to perform thereby allowing more position measurements to be made per second, and hence improving the system's ability to respond to movements of the stylus 5 relative to the sensor PCB 13.
  • the excitation power is not concentrated at any particular frequency but at multiple excitation frequencies over the frequency band of interest. As a result, the power is more evenly distributed across the frequencies of interest, which is desirable since it minimises the power required to cover the frequency range of operation with sufficient energy to detect the stylus 5 with significant signal level.
  • Figure 18 is a block diagram illustrating in more detail the components of the stylus frequency determining unit 259 that would be used in the above embodiment in which three excitation-detection sequences are used with different excitation frequencies, in which the offsets are removed and in which the frequency of the stylus 5 is determined using the above-described amplitude method.
  • the in-phase and quadrature phase signal levels are initially stored in a buffer 409. Once the in-phase and quadrature phase components from the three excitation-detection sequences have been stored in the buffer 409, pairs of in-phase components and pairs of quadrature phase components are passed to an offset elimination unit 411 which calculates the following offset compensated in-phase and quadrature phase values:
  • I(F 2 -F 1 ) I(F 2 )-I(F 1 ) (19)
  • These offset compensated in-phase and quadrature phase values are then passed to the signal amplitude determining unit 401 which calculates A e (F 2 -F ⁇ .) and A e (F 3 - F 2 ) as described above.
  • These amplitude measures are then passed directly to the logarithm determining unit 405 which calculates the logarithm calculation defined in the equation (18) above.
  • the logarithm value output by the logarithm determining unit 405 is then passed to the transformation unit 407' which uses an appropriate linear transformation to transform the logarithm value into a measure of the resonant frequency of the stylus 5.
  • the linear transformation used will be similar to the one used in the transformation unit 407 shown in Figure 16, except with different gain and offset parameters.
  • the signal measurements from three excitation-detection sequences were used to estimate the resonant frequency of the stylus 5. It is possible to extend this approach using more than three excitation-detection sequences with different excitation frequencies and to combine the amplitudes, for, example, using equation (14) above.
  • the offsets may not remain constant across the different excitation frequencies, for example if they depend on integration time. In this case, the above approach may not result in ideal cancellation of the offsets. This could be overcome by weighting each of the in-phase and quadrature phase components depending on the integration time used for the different excitation-detection sequences.
  • an additional fixed offset may be added to I(F 2 -F X ). In this case, the majority of the offset will be removed by the subtraction of I(F ⁇ ) from I(F 2 ) because the integration times are typically similar. The additional offset that is subtracted then removes any additional offset due to, for example, different integration times . Similar constant offsets may be subtracted from I(F 3 -F 2 ) and the corresponding quadrature phase components .
  • the phase method and the amplitude method have been described above for estimating the resonant frequency of the stylus 5 from the in-phase and quadrature phase signals produced by the processing electronics.
  • the amplitude method can provide a coarse estimate of the resonant frequency of the stylus 5 over a relatively large range of frequencies and the phase method can provide a more accurate estimate but one which is ambiguous over the same frequency range. It is therefore possible to use the results of the amplitude method to resolve the ambiguity associated with the phase method.
  • the pair of in-phase components I(F 3 -F 2 ) and I(F 2 -F ⁇ ) may be combined together and the pair of quadrature phase components Q(F 3 -F 2 ) and CKF Z -F- L ) may be combined together as follows:
  • I(F 3 ,F 25 F 1 ) I(F 3 -F 2 )-I(F 2 -F 1 ) (23)
  • Figure 19a shows how these combined in-phase and quadrature phase measurements from the three excitation- detection sequences vary with the resonant frequency of the stylus 5.
  • Figure 19a also shows how the combined phase measurement ( ⁇ (F 3 ,F 2 ,F ⁇ ) ) varies with the resonator frequency. As shown by the sawtooth nature of this phase plot, the phase measurement is ambiguous over the frequency detection range of interest. This ambiguity can then be resolved using the combined amplitude measure g(F 3 ,F 2 ,F ! ) calculated above in equation (18), using the following calculation:
  • a ⁇ - (F 3 ,F 2 ,F 1 ) ⁇ (F 3 ,F 2 ,F 1 ) + where m 3 is a constant such that the rate of change of g(F 3 ,F 2 ,F ! ) multiplied by m 3 with the resonant frequency of the stylus 5 is nominally equal to the rate r>f change of ⁇ (F 3f F 2 ,F 1 ) with the resonant frequency of the stylus
  • FIG. 19b is a plot illustrating the way in which both ⁇ (F 3 ,F 2 ,F ! ) and g(F 3 ,F 2 ,F_) vary with the resonant frequency of the stylus
  • the phase measure ⁇ (F 3 ,F 2 ,F ⁇ ) repeats every ⁇ radians. This is because the coupling factor between the stylus 5 and each of the sensor windings can invert depending on the position of the stylus 5 relative to the sensor windings . This is because of the "figure of eight" arrangement of the sensor windings. In embodiments where the coupling factor between the stylus 5 and the sensor winding does not invert, the phase measure will repeat every 2 ⁇ radians. Therefore, in such embodiments, the ⁇ term in equation (26) should be replaced by 2 ⁇ .
  • the combined amplitude and phase measurement generated using equation (26) will vary linearly with the resonant frequency of the stylus 5 over the desired frequency range of interest. It may therefore be transformed into an estimate of the resonant frequency of the stylus 5 through an appropriate linear transformation like those defined in equations (12) and (13) above.
  • Figure 20 is a block diagram illustrating in more detail the components of the stylus frequency determining unit 259 that would be used in the above embodiment in which the amplitude measure is used to resolve the phase ambiguity associated with the phase measure of the resonant frequency of the stylus 5.
  • the offset compensated in phase and quadrature phase values are passed not only to the signal amplitude determining unit 401 but also to a signal combination unit 415 which calculates and stores in the buffer 417 the combined in-phase and quadrature phase values I(F 3 ,F 2 ,F !
  • phase determining unit 419 which calculates a measure of the electrical phase angle ( ⁇ (F 3 ,F 2 ,F ⁇ ) ) in accordance with equation (25) above.
  • phase measure ( ⁇ (F 3 ,F 2 ,F ! ) ) determined by the phase determining unit 419 and the amplitude measure (g(F 3 ,F 2 ,F ! ) ) determined by the signal amplitude determining unit 405 are input to a phase ambiguity resolving unit 421 which calculates the above described combined amplitude and phase measurement (A ⁇ (F 3 ,F 2 ,F ! ) ) in accordance with equation (26) above.
  • This combined amplitude and phase measurement is then output to a transformation unit 407" which transforms the measurement into an estimate of the resonant frequency of the stylus 5 using an appropriate linear transformation like those defined in equations (12) and (13) above.
  • the processing electronics 59 may be arranged to identify the sensor winding which outputs the largest in-phase and quadrature phase signal levels which it then uses in the above calculations.
  • the in-phase and quadrature phase measurements from the x-direction sensor windings may be combined and/or the in-phase and quadrature phase measurements from the y-direction sensor windings may be combined, with the resonant frequency estimation and offset compensation being performed using the combined in-phase and quadrature phase measurements.
  • Similar combinations may also be performed for the in- phase and quadrature phase components from the y- direction sensor windings 35 and 37.
  • the above-described amplitude method may then be used to estimate the resonant frequency of the stylus 5.
  • phase angle ( ⁇ ) For the purposes of the above-described phase method, it is possible to determine the phase angle ( ⁇ ) from whichever of the sensor windings provides the most reliable measurements, for example by determining which has the greater signal strength. For example, for the x-direction sensor windings, the following measures may be determined:
  • the in-phase and quadrature phase measurements to be used in the phase method may then be determined from the following:
  • the in-phase and quadrature phase measurements from the sensor winding with the lower measure of signal strength may be included in the phase measurement by combining it with MXI and MXQ above. If the in-phase and quadrature phase components of the sensor winding with the lower measure of signal strength are defined as LXI and LXQ respectively, then these are defined as follows:
  • RSIGNSC is the relative sign between the coupling factors between the sin x sensor winding 31 and the stylus 5 and the cos x sensor winding 33 and the stylus 5, which may be determined from:
  • RSIGNSC may be calculated from:
  • RSIGNSC SIGN[MXI.LXI+ MXQ.LXQ] ⁇ 35 > or from
  • RSIGNSC 1if(
  • in-phase and quadrature phase measurements obtained from the y- direction sensor windings 35 and 37 may be performed on any set of in- phase and quadrature phase measurements from any two sensor windings.
  • the in-phase and quadrature phase measurements obtained from different excitation-detection sequences using different excitation frequencies may be combined in a similar manner to allow improved estimation of the phase and therefore of resonant frequency of the stylus 5.
  • the in- phase components from the x-direction sensor windings can be combined with the in-phase components from the indirection sensor windings and similarly the quadrature phase components from the x-direction sensor windings can be combined with the quadrature phase components from the y-direction sensor windings.
  • the above combination procedure may also be used in embodiments which use many sensor windings, such as those used in US 4878553.
  • the sensor winding having the maximum signal strength can be determined and the signals from additional sensor windings near the sensor winding with the maximum signal strength would then be combined in the above manner.
  • the position of the stylus 5 relative to the sensor PCB 13 was determined solely on the basis of the in-phase signal components from the sensor windings. Whilst this approach yields the most accurate results when the excitation frequency matches the resonant frequency of the stylus 5, this may not be the case. Therefore, instead of using only the in-phase components, the processing electronics 59 can be arranged to use the in-phase or the quadrature phase component, depending on which one has the largest signal level. Alternatively, the two components from the same sensor winding can be combined. This may be achieved, for example, by using the length of the corresponding (I,Q) vector defined by these components resolved at a phase angle PR, which can be selected for some optimum system performance.
  • the phase angle PR may be set equal to the electrical phase angle ⁇ calculated above, which results in maximum signal strength.
  • the phase angle PR may be set equal to the phase angle ⁇ plus some constant, where the constant is chosen to minimise the effect of some known error source such as eddy currents flowing in the electrostatic screen 17 provided over the sensor PCB 13.
  • the following equation may be employed to determine the length of the vector ( I(F 3 ,F 2 ,F ⁇ ) , Q(F 3 F 2 ,Fi)) along a line at angle A ⁇ (F 3 ,F 2 ,F 1 ) ) to the origin:
  • phase angle PR may be selected from one of a possible number of angles, with the appropriate phase angle being chosen which is closest to the estimated electrical phase angle A ⁇ .
  • the advantage of this approach is that it can simplify the calculation of equation (37) above, by choosing the different phase angles to be ones that are easy for a micro-controller to calculate with a limited instruction set and speed.
  • multiplication by 1 is trivial and by 1.25 is relatively straightforward since it requires a shift by two bits in binary representation followed by one addition.
  • the position of the stylus 5 relative to the sensor PCB 13 can be estimated from:
  • the resonant frequency of the stylus 5 has been estimated using one of the above approaches, it is possible to perform an additional excitation-detection sequence with the excitation frequency equal to this frequency estimate, in order to improve the estimate of the resonant frequency of the stylus 5 and to improve the accuracy of the determined position of the stylus 5.
  • the number of excitation and reception cycles i.e. N ⁇ x and N RX
  • the estimate of the resonant frequency of the stylus 5 may then be corrected as follows:
  • RFE F EX -GN.PN+ O d (40) where RFE is the final estimate of the resonant frequency of the stylus 5, F ex is the final excitation frequency, GN is the rate of change of the phase estimate PN with resonator frequency and 0 d is an offset to compensate for system phase and frequency offsets and errors .
  • the techniques described above for estimating the resonant frequency of the stylus 5 and for estimating the position of the stylus 5 relative to the sensor PCB 13 may fail if the resonant frequency of the stylus 5 changes significantly between two or more of the excitation-detection sequences . This may be acceptable providing false frequency and position data is not reported to the PDA electronics via the interface 77. Therefore, the position and frequency data is preferably passed to the control unit 253 which performs tests to verify that the data is correct. In the case of a two-stage measurement process such as the one used in the first embodiment, one possible test is to verify that position and frequency calculations for each stage are within a predetermined tolerance of each other. A reasonable tolerance will have to be provided since the position and frequency measurements obtained from the first stage will be relatively coarse and the resonator position may change during the measurement process.
  • a disadvantage of the two-stage measurement process outlined above is that several excitation-detection sequences are required. Further, the first stage is relatively wasteful of both time and power because most of the time the resonant frequency of the stylus 5 will not change between position samples especially at high sample rates of approximately 100 Hz (i.e. 100 position measurements per second) . This problem may be overcome simply by omitting the first stage once the resonant frequency of the stylus 5 has been reliably estimated. The resonant frequency of the stylus 5 may still be updated each time using equation (40) above, enabling the system to track subsequent changes to the resonant frequency of the stylus 5.
  • the stylus 5 approaches the sensor PCB 13 rapidly and position and frequency estimates are determined once at some distance unclicked then a second time clicked so that the frequency changes significantly, the increase in amplitude of the signals caused by the closer proximity of the stylus 5 to the sensor PCB 13 will be offset by the reduction in the amplitude due to the difference in frequency between the resonant frequency of the stylus 5 and the excitation frequency. Therefore, the estimated amplitude measure A e might not change significantly between the two measurements. Further, the phase ( ⁇ ) may have wrapped twice so that the phase and position remains the same between the two measurements. The system may therefore be unable to tell the difference between the position and frequency data from the two measurements.
  • This ambiguity can be resolved by arranging for the processing electronics 59 to have processing channels with two or more different rates of change of phase with frequency.
  • the mixers 69 and 71 could be repeated, with the second set driven by a second RX gate signal with a different duration.
  • Each sensor winding would then be connected to four input channels, comprising I and Q at two different RX gate durations.
  • the in-phase and quadrature phase components output from the channels with the first RX gate duration can be labelled IA and QA and the in-phase and quadrature phase components output f om the channels driven by the RX gate with the second duration may be labelled IB and QB.
  • Figure 21a illustrates the way in which these four in- phase and quadrature phase components vary with the resonant frequency of the stylus 5 for the case where the number of excitation cycles (i.e. N ⁇ x ) is set at 16 and the number of receive cycles (i.e. N RX ) is set at 16 for the A channels and 24 for the B channels.
  • Figure 21b illustrates the calculated amplitudes (A e (A) ,A e (B) ) and phases (P e (A) ,P e (B) ) for the A and B channel pairs.
  • Figure 21b also illustrates the phase difference measurement P e (B-A) which is defined as follows:
  • P e (B-A) P e (B)-P e (A)+*' ,- IIf(',-P e ('-B)--P e ('A ⁇ ") ⁇ , ⁇ -- ⁇ * till e • lse0-', -'f,"1If- ⁇ ',-P e ('- ⁇ )--P e ('A ⁇ ") ⁇ ,- ⁇ ⁇ * mind e • lse0- ⁇ , (4 )
  • P e (B-A) is unambiguous over a much greater frequency range than either P e (A) or P e (B).
  • the resonant frequency of the stylus 5 can then be estimated from the value of P e (B-A) and an appropriate linear transformation function.
  • the frequency estimate based on P e (B-A) is relatively inaccurate.
  • the accuracy may be improved by combining P e (B-A) with P e (B) or P e (A), or some function of the two, whichever yields the best accuracy. That combination may be performed in a similar way to the way in which ⁇ (F 3 ,F 2 ,F 1 ) was combined with g(F 3 ,F 2 ,F x ) using equation (26) above.
  • This approach may therefore yield an estimate for the resonant frequency of the stylus 5 for resonator frequencies substantially beyond the wrapping points of P e (A) or P e (B) individually. If the frequency difference between the excitation frequency and the resonant frequency of the stylus 5 is sufficient, P e (B-A) may wrap and there will be an ambiguity. However, at this point the amplitude will be very substantially lower than before, and the signal level comparator 255 will be able to detect that the data is inconsistent between the two measurements. In this case, the original two-stage measurement process may be used the next time a position and frequency estimate is to be determined.
  • the sensor windings used in the first embodiment described above include two sensor windings for measuring in the X-direction (SX and CX) and two for the Y-direction (SY and CY).
  • the signals from the SX and CX sensor windings may be sufficient to determine phase with the stylus 5 at any point of interest over the sensor board. This is also true for the signals from the SY and CY sensor windings.
  • FIG 22 is a schematic functional block diagram illustrating the excitation and processing electronics used in such an embodiment.
  • the digital processing and signal generation unit 59 includes two RX gate controllers 67-1 and 67-2 which generate two different RX gate signals of different durations which are applied to the switches 63 and 65.
  • the RX gate signal from the first RX gate controller 67-1 is applied to switches 63-1 and 65-1.
  • Switch 63-1 controls the time over which the in-phase mixing signal is applied to the mixers 69-1 and 69-3 for mixing with the signals induced in the X-direction sensor windings 31 and 33.
  • Switch 65-1 controls the time over which the quadrature phase mixing signal is applied to mixers 69-2 and 69-4 for mixing with the signals induced in the X- direction sensor windings 31 and 33.
  • the RX gate signal generated by the second RX gate controller 67-2 is applied to switches 63-2 and 65-2. As shown in Figure 22, these switches control the time over which the in-phase and quadrature phase mixing signals generated by the frequency generator 53 are applied to the signals induced in the Y-direction sensor windings 35 and 37 via the appropriate mixers 69-5 to 69-8.
  • the in-phase and quadrature phase components generated by the integrators 71 are then passed to the digital processing and signal generation unit 59, via the A to D converter 73, which then determines the resonant frequency of the stylus 5 and the position of the stylus 5 relative to the sensor board in the manner described above.
  • in-phase and quadrature phase signals from channels having different rates of change of phase with resonant frequency of the stylus 5 were used to estimate the resonant frequency and position of the stylus 5.
  • the channels were arranged to have different rates of change of phase with resonant frequency by using RX gate signals with different duration for the different channels.
  • RX gate signals with different duration for the different channels.
  • a similar result may be achieved other than by varying the RX gate signals.
  • the time between the end of the excitation cycles and the beginning of the RX gate signal may be modified or the gain of the respective channels may be modulated differently as a function of time during the integration process.
  • Another option is to employ two sets of mixer signals for the two sets of channels, where each set is at a different frequency.
  • the approximate resonant frequency of the stylus 5 may then be calculated from the relative amplitude of signals from each set of channels, and this information may be combined with phase information from one or both sets to determine resonator frequency more accurately and without phase wrapping.
  • a standby state is required which consumes minimum power and which is used only to detect the presence of the resonant stylus 5 and to "wake up" the PDA electronics from the standby state when an approaching stylus 5 is detected.
  • the stylus detection strategies described above are optimised for accurate position and frequency detection, but generally require more than one excitation-detection sequence and are therefore relatively power-hungry.
  • a single excitation-detection sequence may be employed with sufficiently broad frequency range to detect any resonant stylus 5 of interest.
  • the effects of offset may be limited by comparing successive measurements, rather than observing when some combination of a current measurement exceeds a predetermined threshold. If the system detects a significant difference between the current data and historical data, for example the immediately previous data, a flag may be set to indicate that the stylus 5 has appeared. As those skilled in the art will appreciate, this approach does not yield an accurate estimate of the resonant frequency of the stylus 5 or an accurate estimate of its position, but it is sufficient to wake up the host system (i.e. the PDA electronics) and initiate an alternative stylus sensing mode.
  • the host system i.e. the PDA electronics
  • the system operates in one of three modes: i) standby mode - in which single excitation detection sequences are carried out twice a second until a significant change in signal levels is observed, i) stylus frequency acquisition mode - which is entered when a significant change in signal levels is observed and in which the three frequency excitation-detection process (described above with reference to Figures 19 and 20) is employed to determine the resonant frequency of the stylus 5 and its current position.
  • this three frequency excitation-detection process is repeated at a rate of 5 times per second if the signals induced in the sensor windings by the stylus 5 are below some first threshold level.
  • a tracking mode - which is entered if the signals induced in the sensor windings by the stylus 5 are above a third threshold which is greater than the first threshold and in which the above-described modified phase method (described with reference to Figure 21) is used to determine the resonant frequency and position of the stylus 5.
  • this tracking mode of operation is carried out 100 times per second until either the level of the signals induced in the sensor windings by the stylus 5 falls below some fourth threshold level (between the first and third threshold levels) or the system loses track of the resonant frequency of the stylus 5 (for example due to rapid change in frequency) . In this case, the system reverts back to the acquisition mode discussed above.
  • Figure 23 is a block diagram illustrating in more detail the components of the stylus frequency determining unit 259 that would be used in this preferred embodiment in which the system operates in one of the above three modes.
  • the control unit 253 controls the excitation and receive control unit 57 to perform a single excitation-detection sequence twice a second until the signal level comparator 255 detects a significant change in the signal level.
  • the control unit 253 controls the excitation and receive control unit 57 to perform the three sequence excitation-detection process in accordance with the stylus frequency acquisition mode discussed above.
  • the in-phase and quadrature phase signal measurements received from the buffer 251 are then passed by the control unit 353 to the stylus frequency determining unit 259 shown in more detail in Figure 23.
  • these in-phase and quadrature phase values are passed via the switch 408 to the buffer 409 where the in-phase and quadrature phase values from the three excitation-detection sequences are stored.
  • the subsequent processing performed on these in- phase and quadrature measurements are the same as those described with reference to Figure 20 and will not be described again.
  • control unit 253 determines that the tracking mode is to be entered, it sends an appropriate control signal to the stylus frequency determining unit 259 in order to change the position of the switch 408 so that the in-phase and quadrature phase measurements obtained from the x-direction sensor windings are passed to the x-direction I and Q combination unit 421 and the in-phase and quadrature phase measurements from the y-direction sensor windings are passed to the y-direction I and Q combination unit 423.
  • the x-direction combination unit 421 is where the in-phase measurements from the two x- direction sensor windings are combined to give a single combined in-phase measurement and where the two quadrature phase measurements from the two x-direction sensor windings are combined to give a single quadrature phase measurement.
  • these combined in-phase and quadrature phase measurements are obtained in accordance with equation (33) above.
  • the y-direction in-phase and quadrature phase combination unit 423 performs a similar combination on the in-phase and quadrature phase measurements obtained from the two y- direction sensor windings.
  • the combined in-phase and quadrature phase values are then passed to a second phase determining unit 425 which calculates a value (P e (A))for the electrical phase angle ( ⁇ ) using an inverse arc tangent function.
  • P e (A) a value for the electrical phase angle ( ⁇ ) using an inverse arc tangent function.
  • a similar inverse arc tangent function is performed by the third phase determining unit 427 on the combined in-phase and quadrature phase measurements obtained from the y-direction combination unit 423, to provide a second value (P e (B)) for the electrical phase angle ( ⁇ ).
  • the two measurements of the electrical phase angle ( ⁇ ) are then passed to a first phase combination unit 429 which calculates P e (B-A) defined in equation (41) above, which is then output to a second phase combination unit 431 where it is combined with the original phase estimate determined by the third phase determining unit 427.
  • this combination is performed in accordance with the calculations defined by equation (26), except using P e (B- A) in place of g(F 3 ,F 2 ,F x ) and P e (B) in place of ⁇ (F3 F 2f F ⁇ ) «
  • This corrected phase value is then passed to a second transformation unit 407'" where the phase estimate is converted into an estimate of the resonant frequency of the stylus 5 through an appropriate linear transformation.
  • the PDA electronics may only require position data when the stylus is in the clicked state. If the frequency estimate generated by the acquisition mode is within a first set of frequency limits, determined to test for the unclicked state and including an allowance for possible frequency error of the electronics, it is not necessary to waste power by proceeding to the tracking mode in order to refine the frequency and position estimate, since it is already clear that the stylus 5 is in the unclicked state. However, if it can not be d.etermined that the stylus 5 is unclicked then the system would enter the tracking mode to determine more accurately whether or not the stylus 5 is clicked.
  • information generated by one excitation- detection process may be used to improve the performance of the following excitation-detection process.
  • a measurement of signal level from a current excitation-detection sequence may be used to optimise the sensitivity for the next excitation-detection sequence. That optimisation may take the form of a modification to the excitation power level, integrator gain and the number of excitation and receive cycles (i.e.. N ⁇ x and N RX ), SO that the ratios of signal to noise and signal to errors are maintained at an acceptable level. For example, if the amplitude detected by a current excitation-detection sequence were below a certain threshold, the next excitation-detection sequence could be performed with an increased power level.
  • a hand-held personal digital assistant which includes an x-y digitising tablet which operates with a resonant stylus .
  • Various novel features of the digitiser windings, the stylus and the processing electronics have been described which make the system suited for such low cost high volume applications.
  • the skilled reader will appreciate that many of the novel aspects of the system described are independent of each other.
  • the stylus described above can operate with the prior art digitiser windings described in US 4878553 or W098/58237 and the digitiser windings described above can operate with the prior art stylus, such as those described in US, 5565632, or with any other prior art magnetic field generating or altering device.
  • an excitation operation was performed followed by a detection operation.
  • the detection process it is not essential for the detection process to be performed after the excitation process.
  • the detection process may begin before the excitation process has ended, although this is not preferred due to potential coupling between the excitation winding and the sensor windings, which may induce significant errors in the measurements.
  • the signals induced in each sensor winding were mixed with phase quadrature mixing signals.
  • the signal induced in each sensor winding may be mixed with more than two mixing signals with different mixing phases.
  • the resonant frequency of the stylus can still be estimated using the above- described amplitude method by determining, for example, the square-root of the sum of the squared signal levels from each phase, to replace the sum of the squares of the I and Q data.
  • the calculation of phase could be performed by resolving the appropriate signals into two orthogonal phases, followed by an inverse tangent in the normal way. An example of this approach is described in WO 99/18653.
  • the signals induced in the sensor windings were passed through respective processing channels comprising a mixer and an integrator.
  • the mixing and integration process may be performed in the,, digital electronics, with the raw sensor signals being fed directly into the analogue-to-digital converter.
  • such an embodiment requires more complex digital electronics.
  • digital electronics were provided for processing the signals generated in the sensor windings by the resonant stylus.
  • This processing electronics may be formed from separate electronic components or as a single integrated circuit.
  • the processing electronics is preferably programmable by the host electronics so that the equations and equation coefficients used to manipulate the input signal levels can be modified if desired.
  • the processing electronics can use any of the above described techniques for estimating the resonant frequency of the resonant stylus, with the technique to be used currently being programmed or controlled by the host electronics .
  • the processing electronics is also programmed with tilt correction data for correcting the determined x-y position measurements for the effects of stylus tilt.
  • tilt correction data for correcting the determined x-y position measurements for the effects of stylus tilt.
  • tilt correction data is stored within the processing electronics. The determined x-y position is then used to address this tilt correction data to identify the correction to be applied. The appropriate correction is then applied to the position measurement and then the corrected measurement is passed to the host electronics.
  • separate sets of tilt correction data are stored outside of the processing electronics, with each set being associated with a different angle and orientation (e.g. left handed user or a right handed user) of tilt, with the set of correction values most appropriate to the current user being downloaded and stored in the digitising electronics.
  • the different sets of tilt correction data may be stored within the host or within some external computer and downloaded when required.
  • the resonant frequency of the stylus was estimated and then used to determine whether or not the stylus was clicked or unclicked.
  • the above system may be adapted to output a number representing the resonator frequency and therefore indicative of the pressure applied to the tip. This information may be used, for example, to control the thickness of lines drawn on a display.
  • the stylus may be designed so that its resonant frequency changes with more than just pressure applied to the tip.
  • one or more switches may be provided on the side of the stylus which may be actuated by a user in order to change the resonant frequency of the stylus.
  • the digital processing and signal generation unit knows how the resonant frequency of the stylus will change, the status of the stylus can be determined and reported to the appropriate host system (e.g. the PDA electronics).
  • the stylus may include two resonators having different resonant frequencies, with one resonator being provided at a writing end of the stylus and one at an erasing end of the stylus .
  • the digital processing and signal generation unit can then determine the frequency of the resonator in proximity with the sensor PCB and therefore whether the user requires a writing or an erasing action, depending on the resonant frequency that is detected.
  • a single in-phase measurement was obtained using equation (23) and a single quadrature phase measurement was obtained using equation (24).
  • An inverse arc tangent function was then performed on these two in-phase and quadrature phase measurements in accordance with equation (25).
  • a measure of the electrical phase ( ⁇ ) can be determined for I(F 3 -F 2 ) and Q(F 3 -F 2 ) and for l(F 2 -F ⁇ ) and Q(F 2 -F ! ), with the two phase measures then being combined to provide a single phase estimate.
  • this alternative approach requires the calculation of two inverse tangent functions.
  • the offset compensation techniques described above with reference to Figure 17 are for fixed excitation and detection phases which do not change across the di ferent excitation frequencies. However, it is possible to vary the phase of either the excitation or the mixing signals or both for each excitation-detection process. In this case, the difference between excitation frequencies is preferably modified so that the offset cancellation calculations also yield maximum signal level. The frequency difference for optimum signal level may be increased, by suitable choice of excitation and/or mixing signal phase. This approach may therefore be used to extend the range of excitation frequencies, thereby extending the range of possible resonator frequencies which can be detected.
  • the resonator frequency detection techniques described above are appropriate for applications other than stylus sensing.
  • the system could be used in conjunction with the sensor and resonator designs described in WO 99/18653 for determining the position and speed of the rotor of an electric motor.
  • the resonator described in this earlier International application is designed so that its resonant frequency varies with some external parameter which is to be measured, and the most convenient mounting point for an appropriate transducer is the moving rotor, then the resonator frequency detection techniques described above could be used to determine the value of the parameter of interest, without the need for slip rings for connection to the transducer.
  • a transducer could be constructed including an inductance or capacitance that depended on rotor torque.
  • the above techniques for determining the resonant frequency of the resonator may be used to report both position and. motor torque to the motor controller. This approach can therefore yield improvements to product cost by minimising components such as slip rings . and by centralising the electronic processing and signalling into a single controller device.
  • the resonant frequency of the stylus was varied by varying the inductance of the resonant circuit.
  • other techniques may be used to vary the resonant frequency of the stylus.
  • the capacitance and/or the resistance of the resonant circuit may be varied either through pressure applied to the tip of the stylus or through the activation of one or more switches on the stylus.
  • a magnetic washer was provided to increase the change in the resonant frequency of the stylus between its clicked and unclicked states. As those skilled in the art will appreciate, it is not essential to use such a magnetic washer. Further, if a magnetic washer is to be used, it is not essential that the washer is split. However, using a split washer prevents eddy currents being generated in the washer which would generate their own magnetic field which would oppose the magnetic field generated by the resonant circuit. Further, if a washer is to be used, it does not need to be formed as a flat ring with circular inner and outer edges. For example, the washer may be star-shaped, with a star-shaped or square etc. central hole.
  • Figure 14 illustrates an alternative embodiment where the ferrite rod 153 may pass through the magnetic washer 157 in operation.
  • Figure 14a illustrates the alternative stylus design in the unclicked state, with the ferrite rod 153 extending through the inner diameter of the split washer 157.
  • the ferrite rod 153 passes through the washer 157 thereby increasing the gap between the ferrite rod 153 and the split washer 157.
  • the unclicked position of the ferrite rod relative to the coil and the split washer was defined by the distance between the rear face 159a of the nib 159 and the front face 160a of the nib's head 160; and the distance between the first shoulder 167 and the second shoulder 168 of the front body portion 152.
  • the unclicked position may be defined by the thickness of a non-conductive, nonmagnetic spacer.
  • Figure 15 shows the spacer 303 provided between the ferrite rod 153 and the washer 157. The thickness of the spacer 303 can be tightly toleranced compared to dimensions of injection-moulded parts.
  • the ferrite rod 153 position relative to the coil 45 and washer 157 is defined by only one dimension, the distance between the rear face 159a of the nib and the rear face 160b of the nib head 160. That dimension can be tightly controlled since the same side of an injection mould tool can define both faces.
  • the spacer 303 can be omitted, but low unclicked resonator Q-factor may result due to eddy currents in the split washer since the ferrite rod 153 will rest on the split washer 157.
  • the split washer may be manufactured from Permalloy for low cost and ease of handling, or from any other magnetically permeable metals such as Mumetal or spin- melt ribbon.
  • a non-conductive magnetically permeable device for example a ferrite component, may replace the metal washer.
  • the ferrite component may be thicker than the washer since it is easier to manufacture and handle that way.
  • a disadvantage of a ferrite washer component is that the tolerance on its thickness may be significantly greater than that of a thin metal washer.
  • the resonant frequency of each stylus was tested for both the clicked and unclicked states at the time of manufacture, to ensure that the resonant frequencies were within the manufacturing tolerances. If they were not, then the stylus was discarded.
  • the manufacturing step may include the additional step of varying the stylus set-up in order to change the clicked resonant frequency and/or the unclicked resonant frequency. This may be achieved in a number of different ways.
  • Figure 16 schematically illustrates a mechanical approach to varying the clicked and unclicked resonant frequencies of the stylus at the time of manufacture.
  • the resonant frequency is adjusted with a small adjustment in the relative axial position of the ferrite rod 153 and the nib 159. This adjustment is achieved with a pin 301 sliding in a recess 302 in the nib 159.
  • the pin 301 or recess 302 or both may be splined to prevent the pin insertion creating stored pressure that subsequently shifts the position of the pin 301.
  • Glue is preferably applied to the pin 301 or recess 302 before assembly.
  • a similar approach is to perform an initial test of the resonant frequency of the stylus with a pin 301 of known length, then to remove this pin 301 and ferrite rod 153 and to reassemble it with an alternative pin 301 whose length is determined by the results of the initial test.
  • An alternative to this approach is illustrated in Figure 17, where a spacer 303 is added between the nib 159 and the ferrite rod 153, with the thickness of the spacer being determined from the initial test.
  • the initial test may be performed with a stylus with no spacer, or possibly a reference spacer. In both these situations, the algorithm for choosing a spacer or pin component may be performed automatically by the signal detector, processor and display unit of the test apparatus.
  • the height of the rear end face 159a of the nib 159 relative to rear face 160b of the nib head 160 can be reduced by spinning the ferrite rod 153 at high speed and pressing it against the nib 159 until the friction has softened the nib 159 and the ferrite rod 153 has reached the required position.
  • Another alternative is to assemble the stylus 5 with the coil 45 free to slide, requiring a modification to the design of the rear body portion 154 so that it no longer locates the coil 45 against the front body shoulder 167. Initially, the coil 45 would be against the front body shoulder 167. The resonant frequency of the stylus would then be tested with an appropriate test apparatus . The nib 159 would then be forced upward and released and the unclicked frequency measured. While the unclicked frequency is above the desired resonant frequency, this process would be repeated, each time increasing the distance between the coil 45 and the front body head 160. In this way, the unclicked frequency is set to the target value.
  • the coil 45 may then be glued in position, for example, by injecting glue through a small hole in the side of the stylus body. Or, if glue were previously applied to the coil 45, the cure time of the glue may be chosen such that movement during adjustment as described above is possible but further movement after adjustment is prevented.
  • the resonant frequency of the resonator 41 by changing the length of the ferrite rod 153.
  • the top of the ferrite rod 153 may be ground shorter until the resonant frequency reaches the desired target value.
  • an additional length of ferrite 305 may be added to the assembly, to increase the effective length of the ferrite rod 153. Again, the length of this additional . ferrite component 305 would be chosen depending on the results of an initial test of the resonant frequency.
  • the plastic sleeve 155 preferably acts to retain the additional ferrite component 305.
  • the value of the capacitor 43 may be modified, for example by laser trimming.
  • Another option is to vary the number of turns in the coil 45. For example, turns may be added or removed from the coil 45 with the stylus in a test apparatus such as that shown in Figure 10. The coil ends would be left long so that it is possible to wind further turns over the plastic sleeve 155. The operator would add or subtract turns until the resonant frequency of the stylus reached the required target frequency band.
  • the coil 45 may be manufactured with a variable number of turns in order to compensate for the variability in other components such as the capacitor 43.
  • Figure 19 illustrates a plot 325 which shows that the resonant frequency of the resonator 41 varies by +/- 2.5% in response to a variation in the capacitor value of +/- 5%. Therefore, if the number of coil turns is matched to the capacitor value, by first measuring the capacitor value and then matching it with a coil with an appropriate number of turns, the frequency variability can be reduced dramatically.
  • the number of coil turns may be specified from plot 321 illustrated in Figure 20 and results in +/- 0.15% frequency variability, as illustrated in plot 327 shown in Figure 19.
  • an automated machine may perform this selection process.
  • a station of the machine would measure the capacitor's value and would send a signal to a coil winding station specifying how many turns to wind.
  • the capacitor 43 would then be welded to the coil 45.
  • the rear body portion 154 maintains the coil 45 in position.
  • the coil 45 may be fixed in place before the rear body portion 154 is fixed in position.
  • glue may be applied between the plastic sleeve 155 and the front body portion 152.
  • the plastic sleeve 155 acts to prevent glue from flowing onto the ferrite rod 153 and fouling operation.
  • a two-stage pulse-echo measurement process was carried out.
  • three excitation pulses were transmitted and the received signals were integrated over three excitation periods and in the second stage sixteen excitation pulses were transmitted and the received signals were integrated over sixteen excitation periods.
  • the precise number of excitation periods and/or receive periods used may be varied depending on the system design. Further, it is not essential for the number of excitation periods to match the number of receive periods . It is also possible to vary the number of excitation periods without varying the number of receive periods between the first and second measurement stages. Similarly, it is possible to vary the number of receive periods over which the signals are integrated without varying the number of excitation periods during the two measurement stages .
  • the excitation and processing circuitry was formed in the same device as the excitation and sensor windings.
  • the excitation and the processing circuitry may be provided on a remote body from the sensor windings. All that is required is that the resonant stylus be energised by an appropriate energising field and for the signals received in the sensor windings to be transmitted to the processing circuitry.
  • the number of excitation pulses transmitted during the second measurement cycle was fixed at sixteen.
  • the number of excitation pulses used in the second measurement cycle may be varied, depending on the results of the first measurement cycle. For example, if the processing electronics can determine the click state of the stylus from the signals of the first measurement cycle, the second measurement cycle may be adapted to transmit fewer excitation pulses, thereby saving power. If the system can determine the click state of the stylus from the first measurement cycle, then it is not necessary for the system to recalculate the click status of the pen from the signals received in the second measurement cycle.
  • the system can determine the x,y position of the stylus from the signals in the first measurement cycle, then it is not necessary for the system to recalculate that position using the signals from the second measurement cycle. Further, as those skilled in the art will appreciate, if the processing electronics can determine both the resonator state and the resonator's position from the signals in the first measurement cycle, it is not essential to perform the second measurement cycle at all. In this case, an appropriate inhibiting signal may be output to the excitation and receive control unit to prevent the performance of the second measurement cycle.
  • the stylus was designed so that when pressure was applied to the nib of the stylus the resonant frequency increased. As those skilled in the art will appreciate, the stylus may be designed so that the resonant frequency decreases when pressure is applied to the nib. This may be achieved, for example, by placing the coil towards the rear end of the ferrite rod.
  • a biasing spring was provided towards the rear of the stylus.
  • this spring may be replaced by a low. force spring at the nib-end of the inductor coil.
  • the spring may need to be made short and therefore of an undesirably thin wire diameter to ensure a low actuation force for the nib, which adds to the component cost and assembly difficulty.
  • the use of a metal spring at the nib end may adversely interfere with the resonator's magnetics.
  • a plastic spring arrangement could be used instead, but this would be susceptible to creep over time, resulting in a loss of return force.
  • FIG. 31 illustrates the way in which a mobile telephone 351 may be adapted to include a liquid crystal display 355 and underneath the display an x-y set of digitiser windings such as those described above which are operable to sense the position of a resonant stylus 357.
  • the digitising system may be used to allow the user to create, for example, short text messages which can then be sent by the mobile telephone to another party. If the mobile telephone includes, for example, an organiser, then the digitiser can be used to control the inputting, manipulation and outputting of data from the organiser.
  • the digitiser system employed a number of sensor windings, an excitation winding and a resonant stylus.
  • a stylus having either a short-circuit coil or a magnetic field concentrator such as a piece of ferrite
  • lower signal levels would be induced in the sensor windings and the system could not operate in the pulse-echo mode of operation since the non-resonant elements do not continue to "ring" after the excitation signal has ended.
  • a powered stylus could be used with the sensor windings discussed above.
  • the stylus since the stylus has power to generate its own magnetic field, there is no need for the excitation winding, although it may still be provided in order to give a phase reference signal to the stylus .
  • the power to the stylus may be provided either by a battery contained within the stylus or by connecting the stylus, via a lead, to a power source.
  • powered stylus embodiments are possible, they are not preferred since they increase the cost of the stylus and/or they require a lead to the stylus which interferes with the normal use of the device by the user.
  • a single resonant stylus was provided.
  • the system may operate with multiple styluses having different resonant frequencies. Each stylus may then be assigned a different function in the system.
  • the ferrite core was mounted for movement with the tip and the coil was fixed to the housing.
  • the stylus can operate with the ferrite core being fixed relative to the housing and the coil being mounted for movement with the tip.
  • the washer would preferably be mounted for movement with the coil relative to the ferrite core.
  • FIG 32a is a schematic block diagram of a hand-held personal digital assistant 401 having a rectangular display screen 403 and two electronic slider bars 405a and 405b which are positioned over the measurement area of an x-y digitising tablet. These electronic scroll bars 405 are controlled by the interaction of the resonant stylus (not shown) and the digitizer windings mounted on the printed circuit board 407.
  • FIG 32a the lower right hand corner of the printed circuit board 407 is cut away to allow space for three mechanical buttons 409a, 409b and 409c.
  • Figure 32b schematically illustrates the form of one of the windings 411 which forms part of the printed circuit board 407 shown in Figure 32a.
  • This winding 411 was designed using the same design techniques described above in the first embodiment.
  • the winding 411 includes, for example, the flared corners 413a, 413b, 413c and 413d.
  • the winding 411 shown in Figure 32b is schematic in that it does not have any connection points for connecting it to the processing electronics. As those skilled in the art will appreciate, these connection points may be made at any convenient location on the winding 411.
  • a processing channel comprising two mixers and an integrator was provided for each sensor winding.
  • a single processing channel may be used to process the signals induced in two or more of the sensor windings in a time multiplexed manner. As those skilled in the art will appreciate, whilst this reduces the complexity of the processing electronics, it increases the time required to obtain a position measurement.
  • the sensor windings were arranged to have a sensitivity to magnetic field from the resonator which approximately varies as a single period of a sinusoid over the measurement range.
  • the sensor windings may be arranged so that this sensitivity varies through multiple periods of a sinusoid. In this case, the system will have to keep track of the current period in which the resonant stylus is located. Examples of such multiperiod windings can be found in the applicant's earlier International Application W098/58237.
  • Another alternative is that the sensor windings are arranged so that their sensitivity to the magnetic field from the resonator varies through a fraction of a sinusoid over the measurement area. Such an embodiment is particularly useful in applications where the measurement area is rectangular, in order to ensure that the pitch of the x sensor windings and the y sensor windings are the same.
  • the excitation winding was used to energise the resonator and the signals received in the sensor windings were used to identify the resonator position.
  • the sensor windings may be used to energise the resonator and the signals received on the excitation winding used to identify the location of the resonator.
  • either the sensor windings would have to be energised in turn or if the sensor windings are energised together then separate excitation frequencies would have to be applied to each (which would require separate resonant circuits in the resonator which resonate at those frequencies) so that the processing electronics can distinguish the received signals.
  • the system could operate by energising the resonator using one of the sensor windings and then receiving the signal from the resonator on another sensor winding.
  • the way that such a system can operate is described in the applicant's earlier International Application W098/58237.
  • the excitation winding was wound around the outside of the sensor windings.
  • some of the turns of the excitation coil may alternatively be interlaced with the conductors of the sensor windings. This arrangement can also help maintain uniform outer coil field/sensitivity over the entire sensor board, which helps minimise the dynamic range of the sensor system and hence simplifies the design.
  • the sensor PCB which carries the excitation and sensor windings may be manufactured on a flexible printed circuit board.
  • the connecting portion may be extended to form a flexible tail for connecting the coils to the processing electronics.
  • a flexible PCB can also be used to minimise the thickness of the sensor board, e.g. to less than 0.2mm.
  • each of the sensor windings comprises a number of transverse conductors and a number of connecting conductors for connecting the transverse conductors to each other.
  • the transverse conductors for the x-position sensor windings were located substantially in the y- direction whilst those for the y-position sensor windings extended substantially in the x-direction. As those skilled in the art will appreciate, this is not essential, the transverse conductors only have to cross the relevant measurement direction.
  • an electrostatic screen formed from a layer of carbon ink was provided between the sensor PCB and the backlight for the LCD.
  • Other conductive layers may be used such as an evaporated aluminium film coating or a cross-hatched, fishbone or comb-shaped copper layer.
  • the base of the electroluminescent backlight layer 11 can be grounded, then this can effectively act as the electrostatic screen instead.
  • a hand-held personal digital assistant which employs a liquid crystal type display.
  • the above digitiser system can be employed with other types of screen, such as TFT screens and the like.
  • the sensor PCB was located directly underneath the LCD of the hand-held PDA device. As those skilled in the art will appreciate, the sensor PCB does not have to be located underneath the LCD, it can, for example, be located to one side of it. However, if this is the case, then the overall size of the device will have to be larger.
  • each of the sensor windings was formed using multiple turns of conductor.
  • the sensor windings can be formed using a single turn of conductor. However, this is not preferred, since the sensor winding's sensitivity to the magnetic field generated by the resonator is less sinusoidal and the signal levels output are smaller. It is therefore preferred to have as many turns as possible in the sensor windings .
  • one of the phase quadrature windings may be a conventional type of winding having substantially parallel transverse conductors (such as those described in WO 00/33244), with the transverse conductors of the other phase quadrature winding having multiple bends along their length which are designed to compensate for the positional errors of the conventional winding.
  • sensor windings were used which were designed to have an approximate sinusoidal coupling with the resonant stylus, as a result of which the signals output from the sensor windings varied approximately sinusoidally with the position of the stylus relative to the windings.
  • the approach taken to the design of the sensor windings described above is not limited to such "sinusoidal" windings.
  • the technique can be used on any windings which produce an output signal which varies in a non-monotonic fashion with the position to be measured and in which two or more of such sensor windings are used to resolve the ambiguity caused by this non-monotonic characteristic of the windings by appropriate processing of the sensor signals by the processing electronics.
  • the signals induced in the sensor windings were mixed with the excitation signal and a 90° phase shifted version of the excitation signal in order to generate in phase and quadrature phase outputs, from which the electrical phase information of the resonator was determined.
  • other techniques can be used in order to extract this resonator electrical phase information, such as the timing of zero crossings of the resonator signals, although this technique is not preferred because it is sensitive to noise.
  • the sensed signals are to be mixed with phase offset mixing signals, it is not essential that the mixing signals be 90° out of phase. However, this is preferred since it simplifies the measurement of the electrical phase.
  • two-dimensional x-y digitising systems have been described.
  • some aspects of the present invention are not, however, limited to two-dimensional position encoders.
  • some aspects of the present invention can be incorporated into a one-dimensional linear or rotary position encoder.
  • the resonant stylus, the sensor windings or the processing electronics described above could be used in a linear position detector.
  • the signals from a single sensor winding may be used to determine both the electrical phase information and the position information.
  • the resonator was magnetically coupled to the excitation windings and the sensor windings.
  • the above processing electronics may be used in systems where the excitation device and/or the sensing device are capacitively coupled to the resonator.
  • the signals output from the sensor windings were used and position measurements were obtained by performing an arc-tangent calculation.
  • the applicant's earlier International Applications WO98/00921 or WO90/34171 disclose alternative techniques for determining the position information from the signals induced in the sensor windings .

Abstract

L'invention concerne un système de numérisation de x-y à bas coût destiné à être utilisé dans des dispositifs électroniques grand public, tels que des assistants numériques personnels portatifs, des téléphones mobiles, des navigateurs Web et analogue. Le numérisateur comprend un stylet résonnant, un bobinage d'excitation destiné à alimenter le stylet résonnant et un ensemble de bobinages de détection destiné à détecter le signal généré par le stylet, à partir duquel la position x-y du stylet est déterminée. Un nouveau modèle de stylet est décrit avec deux bobinages numérisateurs et de nouveaux circuits d'excitation et de traitement.
PCT/GB2002/005243 2002-03-05 2002-11-22 Detecteur de position WO2003075212A2 (fr)

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GB0422083A GB2403017A (en) 2002-03-05 2002-05-21 Position sensor
AU2002343056A AU2002343056A1 (en) 2002-03-05 2002-11-22 Position sensor

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GB0205116A GB0205116D0 (en) 2002-03-05 2002-03-05 Position sensor
GB0205116.7 2002-03-05
GB0209372.2 2002-04-24
GB0209372A GB0209372D0 (en) 2002-04-24 2002-04-24 Resonator frequency detection method
PCT/GB2002/002387 WO2002103622A2 (fr) 2001-05-21 2002-05-21 Detecteur de position
GBPCT/GB02/02387 2002-05-21
GB0212699A GB0212699D0 (en) 2002-05-31 2002-05-31 Sensor board with feet
GB0212699.3 2002-05-31

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WO2003075212A3 WO2003075212A3 (fr) 2004-03-04

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WO2003075212A3 (fr) 2004-03-04
AU2002343056A8 (en) 2003-09-16
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AU2002343056A1 (en) 2003-09-16
AU2002367738A1 (en) 2003-09-16
US7406393B2 (en) 2008-07-29
WO2003075213A3 (fr) 2005-07-28
AU2002367738A8 (en) 2003-09-16
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GB2403017A (en) 2004-12-22
GB2405942A (en) 2005-03-16

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